The birth of biology: 5th - 4th century BC
The Greek philosophers, voracious in their curiosity, look with interest at the range of living creatures, from the humblest plant to man himself. A Greek name is coined by a German naturalist in the early 19th century for this study of all physical aspects of natural life - biology, frombios (life) andlogos (word or discourse). It is a subject with clear subdivisions, such as botany, zoology or anatomy. But all are concerned with living organisms.
The first man to make a significant contribution in biology is Alcmaeon, living inCrotona in the 5th century. Crotona is famous at the time for its Pythagorean scholars, but Alcmaeon seems not to have been of their school.
Alcmaeon is the first scientist known to have practised dissection in his researches. His aim is not anatomical, for his interest lies in trying to find the whereabouts of human intelligence. But in the course of his researches he makes the first scientific discoveries in the field of anatomy.
The subsequent Greek theory, subscribed to even byAristotle, is that the heart is the seat of intelligence. Alcmaeon reasons that since a blow to the head can affect the mind, in concussion, this must be where reason lies. In dissecting corpses to pursue this idea, he observes passages linking the brain with the eyes (the optic nerves) and the back of the mouth with the ears (Eustachian tubes).
Aristotle may be wrong about the brain being in the heart, but in general he gives a far more complete and well observed account of biology than any other Greek philosopher.
He inaugurates scientific zoology in his reliance on careful observation. He is particularly acute in his study of marine life, having much to say on the habits of fishes, the development of the octopus family, and the nature of whales, dolphins and porpoises. He is also a pioneer in attempting a system ofclassification. Observing an unbroken chain of gradual developments, as the life of plants shades into that of animals, he acknowledges the complexity of the subject and seems almost to glimpse the pattern of evolution.
Aristotle's notes on botany are lost, but many of his observations no doubt survive in the earliest known botanical text - nine booksOn the History of Plantswritten by Aristotle's favourite pupil,Theophrastus.
Writing in about 300 BC, Theophrastus attempts to classify plants, as well as describing their structure, habits and uses. His remarks are based on observations carried out in Greece, but he also includes information brought back from the newHellenisticempire in the Middle East, Persia and India, resulting from the conquests of Alexander the Great.
The influential errors of Galen: 2nd century AD
The newly appointed chief physician to the gladiators in Pergamum, in AD 158, is a native of the city. He is a Greek doctor by the name of Galen. The appointment gives him the opportunity to study wounds of all kinds. His knowledge of muscles enables him to warn his patients of the likely outcome of certain operations - a wise precaution recommended in Galen'sAdvice to doctors.
But it is Galen's dissection of apes and pigs which give him the detailed information for his medical tracts on the organs of the body. Nearly 100 of these tracts survive. They become the basis of Galen's great reputation in medieval medicine, unchallenged until the anatomical work ofVesalius.
Through his experiments Galen is able to overturn many long-held beliefs, such as the theory (first proposed by the Hippocratic school in about 400 BC, and maintained even by the physicians ofAlexandria) that the arteries contain air - carrying it to all parts of the body from the heart and the lungs. This belief is based originally on the arteries of dead animals, which appear to be empty.
Galen is able to demonstrate that living arteries contain blood. His error, which will become the established medical orthodoxy for centuries, is to assume that the blood goes back and forth from the heart in an ebb-and-flow motion. This theory holds sway in medical circles until the time of Harvey.
Science's siesta: 8th - 15th century
In the profoundly Christian centuries of the European Middle Ages the prevailing mood is not conducive to scientific enquiry. God knows best, and so He should - since He created everything. Where practical knowledge is required, there are ancient authorities whose conclusions are accepted without question -Ptolemy in the field of astronomy,Galen on matters anatomical.
A few untypical scholars show an interest in scientific research. The 13th-century Franciscan friar Roger Bacon is the most often quoted example, but his studies include alchemy and astrology as well as optics and astronomy. The practical scepticism required for science must await the Renaissance.
Leonardo's anatomical drawings: 1489-1515
In about 1489 Leonardo da Vinci begins a series of anatomical drawings. For accuracy of observation they are far in advance of anything previously attempted. Over the next twenty-five years he dissects about thirty human corpses, many of them at a mortuary in Rome - until in 1515 the pope, Leo X, orders him to stop.
His drawings, amounting to some 750, include studies of bone structures, muscles, internal organs, the brain and even the position of the foetus in the womb. His studies of the heart suggest that he was on the verge of discovering the concept of thecirculation of the blood.
Illustrated books: 16th century
It is a coincidence of great value to biology, in which observation is of prime importance, that theRenaissancerevival of interest in science coincides with the invention of printing. As soon as books can be published withwoodcutillustrations set among printed text, naturalists have not only a large new readership but also the ability to show what they have so carefully observed.
The first to make serious use of this opportunity is a botanist, Otto Brunfels, whose three-volumeHerbarum vivae eicones (Living images of plants) is published in Strasbourg between 1530 and 1540.
Brunfels' pioneering example is soon improved upon by another German botanist, Leonhard Fuchs, whoseHistoria Stirpium(History of plants) is published in Basel in 1542. Fuchs introduces a new accuracy, in his depiction and his verbal description of the plants.
A French naturalist of this period provides a good example of theRenaissanceimpulse to match and perhaps even outdo the classical authors. In 1546 Pierre Belon sets off on a two-year tour of lands round the eastern Mediterranean with the specific purpose of finding and depicting animals and plants described by ancient writers.
Belon's travels and observations are recounted in a succession of illustrated volumes published in Paris during the 1550s - on fishes and dolphins (1551), on conifers (1553), on general Middle Eastern curiosities (1555), on birds (1555) and finally 'portraits of birds, animals, snakes, herbs, trees, men and women of Arabia and Egypt, together with a map of Mount Athos and of Mount Sinai for the better understanding of their religion' (1557).
Belon is an unashamed generalist. Meanwhile a highly specialized volume, the most significant of all the early illustrated scientific works, has been published in Basel in 1543 - bringing to a wide public the discoveries of Vesalius.
Vesalius and the science of anatomy: 1533-1543
A young medical student, born in Brussels and known to history as Vesalius, attends anatomy lectures in the university of Paris. The lecturer explains human anatomy, as revealed byGalen more than 1000 years earlier, while an assistant points to the equivalent details in a dissected corpse. Often the assistant cannot find the organ as described, but invariably the corpse rather than Galen is held to be in error.
Vesalius decides that he will dissect corpses himself and trust to the evidence of what he finds. His approach is highly controversial. But his evident skill leads to his appointment in 1537 as professor of surgery and anatomy at the university of Padua.
In 1540 Vesalius gives a public demonstration of the inaccuracies ofGalen's anatomical theories, which are still the orthodoxy of the medical profession.
Galen did many of his experiments on apes. Vesalius now has on display - for comparison - the skeletons of a human being and of an ape.
Vesalius is able to show that in many casesGalen's observations are indeed correct for the ape, but bear little relation to the man. Clearly what is needed is a new account of human anatomy.
Vesalius sets himself the task of providing it, illustrated in a series of dissections and drawings. He has at his disposal a method, relatively new in Europe, of ensuring accurate distribution of an image in printed form - the art of thewoodcut. His studies inaugurate the modern science of anatomy.
At Basel, in Switzerland, Vesalius publishes in 1543 his great work - De humani corporis fabrica (The Structure of the Human Body). There are seven volumes including numerous magnificentwoodcut illustrations. The book is an immediate success, though naturally it enrages the traditionalists who followGalen. Galen's theories have, after all, the clear merit of seniority. They are by now some 1400 years old.
But for those willing to look with clear eyes, the plates in Vesalius's volumes are a revelation. For the first time human beings can peer beneath their own skins, in these strikingly clear images of what lies hidden.
Attempts at classification: 1583-1704
It is a natural impulse for any academic, confronted by the bewildering array of nature's living forms, to try and establish some degree of order. One of the first to make a successful attempt is Andrea Cesalpino, whose De Plantis of 1583 classifies plants according to the characteristics of their flowers, seeds and fruits.
The Swiss physician and botanist Gaspard Bauhin extends Cesalpino's work in two books (Phytopinax1596, Pinax theatri botanici1623). Both titles mean 'gallery of plants', and Bauhin classifies some 6000 examples. The main significance of his work is that he is the first to arrange plants in separate groups, or genera.
Bauhin's work was the beginning of the binomial (two-name) system which subsequently prevailed in the classification of living organisms. Each is placed in a category, and the classification combines the name of the category with that of the wider group of which the organism is considered to be a member..
These two levels of classification eventually become standardized as the genus and the species. A basic problem of classification within this arrangement is to decide how much apparent variation can be allowed to plants or animals grouped as a single species. This is resolved in the work of the English naturalist John Ray, who makes extensive tours in Europe during the 1660s with his patron Francis Willughby. Their express purpose is to classify all plants and animals.
Ray publishes classifications of birds (1676), plants (from 1682), fishes (1686), land animals (1693) and insects (1705). In their original partnership the plan was for Willughby to undertake the animals and Ray the plants. Willughby dies young, in 1672, and Ray credits him with the text on birds and fishes (though amplifying it himself).
The greatest achievement is Ray's own work on botany. TheHistoria Plantarum(1686-1704) describes some 18,600 plants, categorizing them in ways which hold good today. His most influential decision is defining a species as a group which has a mutual fertility, each member capable of reproducing with any other. Ray's efforts prepare the way forLinnaeus.
Harvey and the circulation of the blood: 1628
A book is published in 1628 which provides one of the greatest breakthroughs in the understanding of the human body - indeed perhaps the greatest until the discovery of the structure of DNA in the 20th century.
The book consists of just fifty-two tightly argued pages. Its text is in Latin. Its title isExercitatio anatomica de motu cordis et sanguinis in animalibus ('The Anatomical Function of the Movement of the Heart and the Blood in Animals'). Its author is William Harvey. In this book he demonstrates beyond any reasonable doubt an entirely new concept. Blood, he shows, does not drift in the body in any sort of random ebb and flow. Instead it is pumped endlessly round a very precise circuit.
Until now it has been assumed that the blood in arteries and the blood in veins are different in kind. It is well known that they are of a different colour, and there have been many theories as to what each supply of blood does.
The most commonly held belief is that arterial blood carries some sort of energy connected with air to the body (not far from the truth), and that blood in the veins distributes food from the liver (less accurate).
By a long series of dissections (from dogs and pigs down to slugs and oysters), and by a process of logical argument, Harvey is able to prove that the body contains only a single supply of blood; and that the heart is a muscle pumping it round a circuit.
This circuit, as he can demonstrate, brings the blood up from the veins into the right ventricle of the heart; sends it from there through the lungs to the left ventricle of the heart; and then distributes it through the arteries back to the various regions of the body.
After much initial opposition, Harvey's argument eventually convinces most of his contemporaries. But there are two missing ingredients. His theory implies that there must be a network of tiny blood vessels bringing the blood from the arterial system to the venous system and completing the circuit. But his dissections are not adequate to demonstrate this. It is not till four years after his death that Marcello Malpighi observes the capillaries.
And Harvey is unable to explain why the heart should circulate the blood. That explanation will have to await the discovery ofoxygen.
Malpighi and the microscope: 1661
Marcello Malpighi, a lecturer in theoretical medicine at the university of Bologna, has been pioneering the use of themicroscope in biology.
One evening in 1661, on a hill near Bologna, he uses the setting sun as his light source, shining it into his lens through a thin prepared section of a frog's lung. In the enlarged image it is clear that the blood is all contained within little tubes.
Malpighi thus becomes the first scientist to observe the capillaries, the tiny blood vessels in which blood circulates through flesh . They are so fine, and so numerous, that each of our bodies contains more than 100,000 kilometres of these microscopic ducts.
With their discovery, the missing link in Harvey'scirculation of the blood has been found. For the capillaries are literally the link through which oxygen-rich blood from the arteries first delivers its energy to the cells of the body and then finds its way back to the veins to be returned to the heart.
Leeuwenhoek and the microscope: 1674-1683
Malpighi's pioneering work with themicroscope is taken further by the Dutch researcher Anton van Leeuwenhoek. Teaching himself to grind lenses to a very high degree of accuracy and clarity (some of them providing a magnification of 300x), he uses a simple microscope with a single lens - in effect a tiny and extremely powerful magnifying glass.
With instruments of this kind he is able to observe phenomena previously too small to be seen. In 1674 he is the first scientist to give an accurate description of red blood corpuscles. In 1677 he observes and depicts spermatozoa in the semen of a dog. In 1683 he provides a drawing of animalculae (or bacteria) seen in saliva and dental plaque.
His discoveries, published for the most part in the Philosophical Transactions of the Royal Society in London (though he himself lives in Delft), vividly suggest the excitement of being the first to wander with such enlarged vision among the minutiae of the animal kingdom.
His account of the common flea follows its development from egg to the practical perfection of its adult anatomy. His researches demonstrate for the first time that the tiniest living things have a life cycle and generative systems like any larger creature.
The Linnaean system: 1735-1758
The Swenish botanist Carl von Linné, or in the Latin version of his name Linnaeus, is an obsessive classifier. Outside his own field of natural history he tries his hand at organizing a system of minerals and even of diseases. But his fame derives from his having finally put in place, at the end of an experimental period lasting nearly two centuries, the method of classification in the plant and animal kingdoms which still prevails today.
In 1735 Linnaeus publishesSystema naturae(System of nature), in which he proposes a system capable of classifying all living things. It is based on the twin categories genus and species, pioneered byBauhin and developed by John Ray.
Linnaeus begins his task by defining the genera into which the species of plants will be divided (Genera plantarum1737). Next, over a much longer period, he assigns some 6000 species of plants to their appropriate genera (Species plantarum1753). He follows this with an updated edition of theGenera in 1754.
Linnaeus' criterion for grouping plants, by the number of their stamens and pistils, has proved misleading and has been revised. But his version of the Binomial systemsurvives intact, applying to animals as effectively as to plants. He proposes the use of genus and species to classify animals in the tenth edition of Systema naturae(1758), listing 4236 species as a preliminary contribution.
Cuvier and paleontology: 1812
William Smithin the late 18th century has used the evidence of fossils in rock strata for the advancement of geology. Georges Cuvier studies the fossils for their own sake, and in doing so founds the science of palaeontology.
His researches concentrate on the fossils of mammals and reptiles found in rocks in the Paris region, with special emphasis on extinct mammals of the tertiary period. His results are published in 1812 in the four volumes of Recherches sur les ossements fossiles des quadrupèdes(Researchs on the fossil bones of quadrupeds).
The discoveries revealed in this pioneering work provide the basis for subsequent theories of evolution, though they do not suggest that explanation to Cuvier himself. Confronted by the remains of extinct species, he concludes that the earth has gone through a series of cycles (which he calls 'revolutions'), corresponding to the observableGeological periods.
Each revolution, he believes, ends in some catastrophe of nature which destroys most of the existing fauna and flora. The survivors are joined by fresh species resulting from a new bout of creation. Subsequent researches by others, unearthing transitional fossils, give weight to the argument for a more gradual or evolutionary process.
The Greek philosophers, voracious in their curiosity, look with interest at the range of living creatures, from the humblest plant to man himself. A Greek name is coined by a German naturalist in the early 19th century for this study of all physical aspects of natural life - biology, frombios (life) andlogos (word or discourse). It is a subject with clear subdivisions, such as botany, zoology or anatomy. But all are concerned with living organisms.
The first man to make a significant contribution in biology is Alcmaeon, living inCrotona in the 5th century. Crotona is famous at the time for its Pythagorean scholars, but Alcmaeon seems not to have been of their school.
Alcmaeon is the first scientist known to have practised dissection in his researches. His aim is not anatomical, for his interest lies in trying to find the whereabouts of human intelligence. But in the course of his researches he makes the first scientific discoveries in the field of anatomy.
The subsequent Greek theory, subscribed to even byAristotle, is that the heart is the seat of intelligence. Alcmaeon reasons that since a blow to the head can affect the mind, in concussion, this must be where reason lies. In dissecting corpses to pursue this idea, he observes passages linking the brain with the eyes (the optic nerves) and the back of the mouth with the ears (Eustachian tubes).
Aristotle may be wrong about the brain being in the heart, but in general he gives a far more complete and well observed account of biology than any other Greek philosopher.
He inaugurates scientific zoology in his reliance on careful observation. He is particularly acute in his study of marine life, having much to say on the habits of fishes, the development of the octopus family, and the nature of whales, dolphins and porpoises. He is also a pioneer in attempting a system ofclassification. Observing an unbroken chain of gradual developments, as the life of plants shades into that of animals, he acknowledges the complexity of the subject and seems almost to glimpse the pattern of evolution.
Aristotle's notes on botany are lost, but many of his observations no doubt survive in the earliest known botanical text - nine booksOn the History of Plantswritten by Aristotle's favourite pupil,Theophrastus.
Writing in about 300 BC, Theophrastus attempts to classify plants, as well as describing their structure, habits and uses. His remarks are based on observations carried out in Greece, but he also includes information brought back from the newHellenisticempire in the Middle East, Persia and India, resulting from the conquests of Alexander the Great.
The influential errors of Galen: 2nd century AD
The newly appointed chief physician to the gladiators in Pergamum, in AD 158, is a native of the city. He is a Greek doctor by the name of Galen. The appointment gives him the opportunity to study wounds of all kinds. His knowledge of muscles enables him to warn his patients of the likely outcome of certain operations - a wise precaution recommended in Galen'sAdvice to doctors.
But it is Galen's dissection of apes and pigs which give him the detailed information for his medical tracts on the organs of the body. Nearly 100 of these tracts survive. They become the basis of Galen's great reputation in medieval medicine, unchallenged until the anatomical work ofVesalius.
Through his experiments Galen is able to overturn many long-held beliefs, such as the theory (first proposed by the Hippocratic school in about 400 BC, and maintained even by the physicians ofAlexandria) that the arteries contain air - carrying it to all parts of the body from the heart and the lungs. This belief is based originally on the arteries of dead animals, which appear to be empty.
Galen is able to demonstrate that living arteries contain blood. His error, which will become the established medical orthodoxy for centuries, is to assume that the blood goes back and forth from the heart in an ebb-and-flow motion. This theory holds sway in medical circles until the time of Harvey.
Science's siesta: 8th - 15th century
In the profoundly Christian centuries of the European Middle Ages the prevailing mood is not conducive to scientific enquiry. God knows best, and so He should - since He created everything. Where practical knowledge is required, there are ancient authorities whose conclusions are accepted without question -Ptolemy in the field of astronomy,Galen on matters anatomical.
A few untypical scholars show an interest in scientific research. The 13th-century Franciscan friar Roger Bacon is the most often quoted example, but his studies include alchemy and astrology as well as optics and astronomy. The practical scepticism required for science must await the Renaissance.
Leonardo's anatomical drawings: 1489-1515
In about 1489 Leonardo da Vinci begins a series of anatomical drawings. For accuracy of observation they are far in advance of anything previously attempted. Over the next twenty-five years he dissects about thirty human corpses, many of them at a mortuary in Rome - until in 1515 the pope, Leo X, orders him to stop.
His drawings, amounting to some 750, include studies of bone structures, muscles, internal organs, the brain and even the position of the foetus in the womb. His studies of the heart suggest that he was on the verge of discovering the concept of thecirculation of the blood.
Illustrated books: 16th century
It is a coincidence of great value to biology, in which observation is of prime importance, that theRenaissancerevival of interest in science coincides with the invention of printing. As soon as books can be published withwoodcutillustrations set among printed text, naturalists have not only a large new readership but also the ability to show what they have so carefully observed.
The first to make serious use of this opportunity is a botanist, Otto Brunfels, whose three-volumeHerbarum vivae eicones (Living images of plants) is published in Strasbourg between 1530 and 1540.
Brunfels' pioneering example is soon improved upon by another German botanist, Leonhard Fuchs, whoseHistoria Stirpium(History of plants) is published in Basel in 1542. Fuchs introduces a new accuracy, in his depiction and his verbal description of the plants.
A French naturalist of this period provides a good example of theRenaissanceimpulse to match and perhaps even outdo the classical authors. In 1546 Pierre Belon sets off on a two-year tour of lands round the eastern Mediterranean with the specific purpose of finding and depicting animals and plants described by ancient writers.
Belon's travels and observations are recounted in a succession of illustrated volumes published in Paris during the 1550s - on fishes and dolphins (1551), on conifers (1553), on general Middle Eastern curiosities (1555), on birds (1555) and finally 'portraits of birds, animals, snakes, herbs, trees, men and women of Arabia and Egypt, together with a map of Mount Athos and of Mount Sinai for the better understanding of their religion' (1557).
Belon is an unashamed generalist. Meanwhile a highly specialized volume, the most significant of all the early illustrated scientific works, has been published in Basel in 1543 - bringing to a wide public the discoveries of Vesalius.
Vesalius and the science of anatomy: 1533-1543
A young medical student, born in Brussels and known to history as Vesalius, attends anatomy lectures in the university of Paris. The lecturer explains human anatomy, as revealed byGalen more than 1000 years earlier, while an assistant points to the equivalent details in a dissected corpse. Often the assistant cannot find the organ as described, but invariably the corpse rather than Galen is held to be in error.
Vesalius decides that he will dissect corpses himself and trust to the evidence of what he finds. His approach is highly controversial. But his evident skill leads to his appointment in 1537 as professor of surgery and anatomy at the university of Padua.
In 1540 Vesalius gives a public demonstration of the inaccuracies ofGalen's anatomical theories, which are still the orthodoxy of the medical profession.
Galen did many of his experiments on apes. Vesalius now has on display - for comparison - the skeletons of a human being and of an ape.
Vesalius is able to show that in many casesGalen's observations are indeed correct for the ape, but bear little relation to the man. Clearly what is needed is a new account of human anatomy.
Vesalius sets himself the task of providing it, illustrated in a series of dissections and drawings. He has at his disposal a method, relatively new in Europe, of ensuring accurate distribution of an image in printed form - the art of thewoodcut. His studies inaugurate the modern science of anatomy.
At Basel, in Switzerland, Vesalius publishes in 1543 his great work - De humani corporis fabrica (The Structure of the Human Body). There are seven volumes including numerous magnificentwoodcut illustrations. The book is an immediate success, though naturally it enrages the traditionalists who followGalen. Galen's theories have, after all, the clear merit of seniority. They are by now some 1400 years old.
But for those willing to look with clear eyes, the plates in Vesalius's volumes are a revelation. For the first time human beings can peer beneath their own skins, in these strikingly clear images of what lies hidden.
Attempts at classification: 1583-1704
It is a natural impulse for any academic, confronted by the bewildering array of nature's living forms, to try and establish some degree of order. One of the first to make a successful attempt is Andrea Cesalpino, whose De Plantis of 1583 classifies plants according to the characteristics of their flowers, seeds and fruits.
The Swiss physician and botanist Gaspard Bauhin extends Cesalpino's work in two books (Phytopinax1596, Pinax theatri botanici1623). Both titles mean 'gallery of plants', and Bauhin classifies some 6000 examples. The main significance of his work is that he is the first to arrange plants in separate groups, or genera.
Bauhin's work was the beginning of the binomial (two-name) system which subsequently prevailed in the classification of living organisms. Each is placed in a category, and the classification combines the name of the category with that of the wider group of which the organism is considered to be a member..
These two levels of classification eventually become standardized as the genus and the species. A basic problem of classification within this arrangement is to decide how much apparent variation can be allowed to plants or animals grouped as a single species. This is resolved in the work of the English naturalist John Ray, who makes extensive tours in Europe during the 1660s with his patron Francis Willughby. Their express purpose is to classify all plants and animals.
Ray publishes classifications of birds (1676), plants (from 1682), fishes (1686), land animals (1693) and insects (1705). In their original partnership the plan was for Willughby to undertake the animals and Ray the plants. Willughby dies young, in 1672, and Ray credits him with the text on birds and fishes (though amplifying it himself).
The greatest achievement is Ray's own work on botany. TheHistoria Plantarum(1686-1704) describes some 18,600 plants, categorizing them in ways which hold good today. His most influential decision is defining a species as a group which has a mutual fertility, each member capable of reproducing with any other. Ray's efforts prepare the way forLinnaeus.
Harvey and the circulation of the blood: 1628
A book is published in 1628 which provides one of the greatest breakthroughs in the understanding of the human body - indeed perhaps the greatest until the discovery of the structure of DNA in the 20th century.
The book consists of just fifty-two tightly argued pages. Its text is in Latin. Its title isExercitatio anatomica de motu cordis et sanguinis in animalibus ('The Anatomical Function of the Movement of the Heart and the Blood in Animals'). Its author is William Harvey. In this book he demonstrates beyond any reasonable doubt an entirely new concept. Blood, he shows, does not drift in the body in any sort of random ebb and flow. Instead it is pumped endlessly round a very precise circuit.
Until now it has been assumed that the blood in arteries and the blood in veins are different in kind. It is well known that they are of a different colour, and there have been many theories as to what each supply of blood does.
The most commonly held belief is that arterial blood carries some sort of energy connected with air to the body (not far from the truth), and that blood in the veins distributes food from the liver (less accurate).
By a long series of dissections (from dogs and pigs down to slugs and oysters), and by a process of logical argument, Harvey is able to prove that the body contains only a single supply of blood; and that the heart is a muscle pumping it round a circuit.
This circuit, as he can demonstrate, brings the blood up from the veins into the right ventricle of the heart; sends it from there through the lungs to the left ventricle of the heart; and then distributes it through the arteries back to the various regions of the body.
After much initial opposition, Harvey's argument eventually convinces most of his contemporaries. But there are two missing ingredients. His theory implies that there must be a network of tiny blood vessels bringing the blood from the arterial system to the venous system and completing the circuit. But his dissections are not adequate to demonstrate this. It is not till four years after his death that Marcello Malpighi observes the capillaries.
And Harvey is unable to explain why the heart should circulate the blood. That explanation will have to await the discovery ofoxygen.
Malpighi and the microscope: 1661
Marcello Malpighi, a lecturer in theoretical medicine at the university of Bologna, has been pioneering the use of themicroscope in biology.
One evening in 1661, on a hill near Bologna, he uses the setting sun as his light source, shining it into his lens through a thin prepared section of a frog's lung. In the enlarged image it is clear that the blood is all contained within little tubes.
Malpighi thus becomes the first scientist to observe the capillaries, the tiny blood vessels in which blood circulates through flesh . They are so fine, and so numerous, that each of our bodies contains more than 100,000 kilometres of these microscopic ducts.
With their discovery, the missing link in Harvey'scirculation of the blood has been found. For the capillaries are literally the link through which oxygen-rich blood from the arteries first delivers its energy to the cells of the body and then finds its way back to the veins to be returned to the heart.
Leeuwenhoek and the microscope: 1674-1683
Malpighi's pioneering work with themicroscope is taken further by the Dutch researcher Anton van Leeuwenhoek. Teaching himself to grind lenses to a very high degree of accuracy and clarity (some of them providing a magnification of 300x), he uses a simple microscope with a single lens - in effect a tiny and extremely powerful magnifying glass.
With instruments of this kind he is able to observe phenomena previously too small to be seen. In 1674 he is the first scientist to give an accurate description of red blood corpuscles. In 1677 he observes and depicts spermatozoa in the semen of a dog. In 1683 he provides a drawing of animalculae (or bacteria) seen in saliva and dental plaque.
His discoveries, published for the most part in the Philosophical Transactions of the Royal Society in London (though he himself lives in Delft), vividly suggest the excitement of being the first to wander with such enlarged vision among the minutiae of the animal kingdom.
His account of the common flea follows its development from egg to the practical perfection of its adult anatomy. His researches demonstrate for the first time that the tiniest living things have a life cycle and generative systems like any larger creature.
The Linnaean system: 1735-1758
The Swenish botanist Carl von Linné, or in the Latin version of his name Linnaeus, is an obsessive classifier. Outside his own field of natural history he tries his hand at organizing a system of minerals and even of diseases. But his fame derives from his having finally put in place, at the end of an experimental period lasting nearly two centuries, the method of classification in the plant and animal kingdoms which still prevails today.
In 1735 Linnaeus publishesSystema naturae(System of nature), in which he proposes a system capable of classifying all living things. It is based on the twin categories genus and species, pioneered byBauhin and developed by John Ray.
Linnaeus begins his task by defining the genera into which the species of plants will be divided (Genera plantarum1737). Next, over a much longer period, he assigns some 6000 species of plants to their appropriate genera (Species plantarum1753). He follows this with an updated edition of theGenera in 1754.
Linnaeus' criterion for grouping plants, by the number of their stamens and pistils, has proved misleading and has been revised. But his version of the Binomial systemsurvives intact, applying to animals as effectively as to plants. He proposes the use of genus and species to classify animals in the tenth edition of Systema naturae(1758), listing 4236 species as a preliminary contribution.
Cuvier and paleontology: 1812
William Smithin the late 18th century has used the evidence of fossils in rock strata for the advancement of geology. Georges Cuvier studies the fossils for their own sake, and in doing so founds the science of palaeontology.
His researches concentrate on the fossils of mammals and reptiles found in rocks in the Paris region, with special emphasis on extinct mammals of the tertiary period. His results are published in 1812 in the four volumes of Recherches sur les ossements fossiles des quadrupèdes(Researchs on the fossil bones of quadrupeds).
The discoveries revealed in this pioneering work provide the basis for subsequent theories of evolution, though they do not suggest that explanation to Cuvier himself. Confronted by the remains of extinct species, he concludes that the earth has gone through a series of cycles (which he calls 'revolutions'), corresponding to the observableGeological periods.
Each revolution, he believes, ends in some catastrophe of nature which destroys most of the existing fauna and flora. The survivors are joined by fresh species resulting from a new bout of creation. Subsequent researches by others, unearthing transitional fossils, give weight to the argument for a more gradual or evolutionary process.
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