THE MOST SALIENT EXCERPTS
(if you don’t want to tackle the whole)
translated by Hugh Elliot
The conditions necessary to the existence of life are all present in the lowest organisations, and they are here also reduced to their simplest expression. It became therefore of importance to know how this organisation, by some sort of change, had succeeded in giving rise to others less simple, and indeed to the gradually increasing complexity observed throughout the animal scale. By means of the two following principles to which observation had led me, I believed I perceived the solution of the problem at issue.
Firstly a number of known facts proves that the continued use of any organ leads to its development, strenthens it and even enlarges it, while permanent disuse of any organ is injurious to its development, causes it to deteriorate and ultimate disappear if the disuse continues for a longer period through successive generations. Hence we may infer that when some change in the environment leads to a change of habit in some race of animals, the organs that are less used die away little by little, while those which are more used develop better, and acquire a vigour and size proportional to their use.
Secondly, when reflecting upon the power of the movement of the fluids in the very supple parts which contain them, I soon became convinced that, according as this movement is accelerated, the fluids modify the cellular tissue in which they move, open passages in them, form various canals, and finally create different organs, according to the state of the organisation in which they are placed…
IMPORTANCE OF THE CONSIDERATION OF AFFINITIES
Among living bodies the name affinity has been given to features of analogy or resemblance between two objects, that are compared in their totality, but with special stress on the most essential parts. The closer and more extensive the resemblance, the greater the affinities. They indicate a sort of kinship between the living bodies which exhibit them; and oblige us in our classification to place these bodies in a proximity proportional to their affinities.
How great has been the progress of natural science since serious attention began to be given to affinities, and especially since their true underlying principles have been determined!
Before this change, our botanical classifications were entirely at the mercy of arbitrary opinion, and of artificial systems of any author. In the animal kingdom, the invertebrate animals comprising the larger part of all known animals were classified into the most heterogeneous groups, some under the name of insects, some under the name of worms; where the animals included are from the point of view of affinity widely different from one another….
We must then be guided everywhere by natural affinities in composing the groups which result by dividing each kingdom into classes, each class into orders, each order into sections or families, each family into genera, and each genus into different species if there is occasion for it.
There is thorough justification for the belief that the complete series of beings making up a kingdom represents the actual order of nature, when it is classified with direct reference to affinities; but, as I have already pointed out, the different kinds of divisions which have to be set up in that series to help us distinguish objects with greater ease do not belong to nature at all. They are truly artificial although they exhibit natural portions of the actual order instituted by nature….
OF SPECIES AMONG LIVING BODIES AND THE IDEA THAT WE SHOULD ATTACH TO THAT WORD
It is not a futile purpose to decide definitely what we mean by the so-called species among living bodies, and to enquire if it is true that species are of absolute constancy, as old as nature, and have all existed from the beginning just as we see them to-day; or if, as a result of changes in their environment, albeit extremely slow, they have not in course of time changed their characters and shape….
Any collection of like individuals which were produced by others similar to themselves is called a species.
This definition is exact: for every individual possessing life always resembles very closely those from which it sprang; but to this definition is added the allegation that the individuals composing a species never vary in their specific characters, and consequently that species have an absolute constancy in nature.
It is just this allegation that I propose to attack, since clear proofs drawn from observation show that it is ill-founded…
We learn from a number of facts that, according as the individuals of one our species change their abode, climate, habits, or manner of life, they become subject to influences which little by little alter the consistency and proportions of their parts, their shape, properties and even their organisation; so that in course of time everything in them shares in these mutations.
In the same climate, very different habitats and conditions at first merely cause variations in the individuals exposed to them; but in course of time the continued change of habitat int he individuals of which I speak, living and reproducing in these new conditions, induces alterations in them which become more or less essential to their being; thus, after a long succession of generations these individuals, originally belonging to one species, become at length transformed into a new species distinct from the first….
The idea of bringing together under the name of species a collection of like individuals, which perpetuate themselves unchanged by reproduction and are as old as nature, involved the assumption that the individuals of one species could not unite in reproductive acts with individuals of another species.
Unfortunately, observation has proved and continues every day to prove that this assumption is unwarranted; for the hybrids so common among plants, and the copulations so often noticed between animals of very different species, disclose the fact that the boundaries between these alleged constant species are not so impassable as had been imagined.
It is true that often nothing results from these strange copulations, especially when the animals are very disparate; and when anything does happen the resulting individuals are usually infertile; but we also know that when there is less disparity these defects do not occur. Now this cause is by itself sufficient gradually to create varieties, which then become races, and in the course of time constitute what we call species….
Thus, among living bodies, nature, as I have already said, definitely contains nothing but individuals which succeed one another by reproduction and spring from one another; but the species among them have only a relative constancy and are only invariable temporarily.
Nevertheless, to facilitate the study and knowledge of so many different bodies it is useful to give the name of species to any collection of like individuals perpetuated by reproduction without change, so long as their environment does not alter enough to cause variations in their habits, character and shape….
Every qualified observer knows that nothing on the surface of the earth remains permanently in the same state. Everything in time undergoes various mutations, more or less rapid according to the nature of the objects and the conditions…everything on the surface of the earth changes its situation, shape, nature and appearance, and even climates are not more stable.
Now I shall endeavour to show that variations in the environment induce changes in the needs, habits and mode of life of living beings, and especially of animals; and that these changes give rise to modifications or developments in their organs and the shape of their parts. If this is so, it is difficult to deny that the shape or external characters of every living body whatever must vary imperceptibly, although that variation only becomes perceptible after a considerable time….
Let us then no longer be astonished that among the numerous fossils found in all the dry parts of the world, and constituting the remains of so many animals which formerly existed, there are so few of which we recognise the living representatives….Naturalists who did not perceive the changes undergone by most animals in course of time tried to explain the facts connected with fossils, as well as the commotions known to have occurred in different parts of the earth’s surface, by the supposition of a universal catastrophe which took place on our globe. They imagined that everything had been displaced by it, and that a great number of the species then existing had been destroyed.
Unfortunately this facile method of explaining the operations of nature, when we cannot see their causes, has no basis beyond the imagination which created it, and cannot be supported by proof.
Local catastrophes, it is true, such as those produced by earthquakes, volcanoes and other special causes are well known, and we can observe the disorder ensuing from them.
But why are we to assume without proof a universal catastrophe, when the better known procedure of nature suffices to account for all the facts which we can observe?
Consider on the one hand that in all nature’s works nothing is done abruptly, but that she acts everywhere slowly and by successive stages; and on the other hand that the special or local causes of disorders, commotions, displacements, etc., can account for everything that we observe on the surface of the earth, while still remaining subject to nature’s laws and general procedure. It will then be recognised that there is no necessity whatever to imagine that a universal catastrophe came to overthrow everything, and destroy a great part of nature’s own works…
ON THE INFLUENCE OF THE ENVIRONMENT ON THE ACTIVITIES AND HABITS OF ANIMALS, AND THE INFLUENCE OF THE ACTIVITIES AND HABITS OF THESE LIVING BODIES IN MODIFYING THEIR ORGANISATION AND STRUCTURE
…[T]o obtain a knowledge of the true causes of that great diversity of shapes and habits found in the various known animals, we must reflect that the infinitely diversified but slowly changing environment in which the animals of each race have successively been placed, has involved each of them in new needs and corresponding alterations in their habits. This is a truth which, once recognised, cannot be disputed. Now we shall easily discern how the new needs may have been satisfied, and the new habits acquired, if we pay attention to the two following laws of nature, which are always verified by observation.
In every animal which has not passed the limit of its development, a more frequent and continuous use of any organ gradually strengthens, develops and enlarges that organ, and gives it a power proportional to the length of time it has been so used; while the permanent disuse of any organ imperceptibly weakens and deteriorates it, and progressively diminishes its functional capacity, until it finally disappears.
All the acquisitions or losses wrought by nature on individuals, through the influence of the environment in which their race has long been placed, and hence through the influence of the predominant use or permanent disuse of any organ; all these are preserved by reproduction to the new individuals which arise, provided that the acquired modifications are common to both sexes, or at least to the individuals which produce the young.
Here we have two permanent truths, which can only be doubted by those who have never observed or followed the operations of nature, or by those who have allowed themselves to be drawn into the error which I shall now proceed to combat.
Naturalists have remarked that the structure of animals is always in perfect adaptation to their functions, and have inferred that the shape and condition of their parts have determined the use of them. Now this is a mistake: for it may be easily proved by observation that it is on the contrary the needs and uses of the parts which have caused the development of these same parts, which have even birth to them when they did not exist, and which consequently have given rise to the condition we find in each animal.
If this were not so, nature would have had to create as many different kinds of structure in animals, as there are different kinds of environments in which they have to live; and neither structure nor environment would ever have varied….
Nature (or her Author) in creating animals, foresaw all the possible kinds of environment in which they would have to live, and endowed each species with a fixed organisation and with a definite and invariable shape, which compel each species to live in the places and climates where we actually find them, and there to maintain the habits which we know in them.
My individual conclusion: Nature has produced all the species of animals in succession, beginning with the most imperfect or simplest, and ending her work with the most perfect, so as to create a gradually increasing complexity in their organisation; these animals have spread at large throughout all the habitable regions of the globe, and every species has derived from its environment the habits that we find in it and the structural modifications which observation shows us….
[N]o point on the surface of the earth ever undergoes variation as to its nature, exposure, high or low situation, climate, etc., etc.; it must then be proved that no part of animals undergoes even after long periods of time any modification due to a change of kind of life or to the necessity which forces them into a different kind of life and activity from what has been customary to them.
Now if a single case is sufficient to prove that an animal which has long been in domestication differs from the wild species whence it sprang, and if in any such domesticated species, great differences of conformation are found between the individuals exposed to such habit and those which are forced into different habits, it will then be certain that the first conclusion is not consistent with the laws of nature, while the second, on the contrary, is entirely in accordance with them.
Everything then combines to prove my statement, namely: that it is not the shape either of the body or its parts which gives rise to the habits of animals and their mode of life; but that it is, on the contrary, the habits, mode of life and all the other influences of the environment which have in the course of time built up the shape of the body and of the parts of animals. With new shapes, new faculties have been acquired, and little by little nature has succeeded in fashioning animals as we actually see them.
Can there be any more important conclusion in the range of natural history, or any to which more attention cannot be paid than that which I have just set forth?
…[L]et us continue the enquiry as to what essentially constitutes life.
Seeing that life in a body results exclusively from the relations existing between the containing parts in an appropriate condition, the contained fluids moving in them, and the exciting cause of the movements, activities, and reactions which take place, we may include what essentially constitutes life in the following definition.
Life, in the parts of any body which possesses it, is an order and state of things which permit of organic movements; and these movements constituting active life result from the action of a stimulating cause which excites them.
This definition of life, either active or suspended, includes all the positive facts which have to be expressed in it, and covers all special cases. It appears to me impossible to add or subtract a single word, without destroying the integrity of the essential ideas contained in it; lastly, it is based on the known facts and observations which have reference to this wonderful natural phenomenon.
To begin with, in this definition active life is kept distinct from that life which, without ceasing to exist, is suspended and appears to be maintained for a limited time without perceptible organic movements; and this, as I shall show, is in accordance with observation.
Then it brings out the fact that no body can possess active life except when the two following conditions are satisfied.
The first is the necessity for a stimulating cause which excites organic movements.
The second is the necessity that a body in order to possess and maintain life should be so ordered in its parts as to possess the property of responding to teh action of the stimulating cause and of producing organic movements…
[W]hen degenerations and disorders of a living body, either in the order or in the state of its parts, are large enough to prevent these parts from yielding to the influence of the exciting cause and producing organic movements, then life is quickly extinguished, and the body hencforth is no longer included among the living….[I]t follows that if in a body any distrubance or degeneration affects the order and state of things which endow it with active life, and if this disturbance is of a nature to prevent the performance of organic movements or their restitution after suspension, the body then loses its life, that is to say, it undergoes death.
A disorder resulting in death may be brought about in a living body through various accidental causes; but nature becomes the necessary cause at the end of a certain period; and in fact, it is a property of life to bring the organs imperceptivly to a condition in which they cannot perform their functions, so that death inevitably ensues.
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