Lamarck, the Founder of Evolution Part 28

"The following fact proves, as regards plants, how a change in any important circ.u.mstance leads to a change in the parts of their organisms.

"So long as _Ranunculus aquatilis_ is submerged in the water, its leaves are all finely incised and the divisions hair-like; but when the stalks of this plant reach the surface of the water, the leaves which grow out in the air are wider, rounded, and simply lobed. If some feet from the same plant the roots succeed in pushing into a soil only damp, without being submerged, their stalks then are short, none of their leaves are divided into capillary divisions, which gives rise to _Ranunculus hederaceus_, which the botanists regard as a species whenever they meet with it.

"There is no doubt that as regards animals important changes in the circ.u.mstances under which they are accustomed to live do not produce alteration in their organs; for here the changes are much slower in operating than in plants, and, consequently, are to us less marked, and their cause less recognizable.

"As to the circ.u.mstances which have so much power in modifying the organs of living beings, the most influential are, doubtless, the diversity of the surroundings in which they live; but besides this there are many others which, in addition, have a considerable influence in the production of the effects in question.

"It is known that different localities change in nature and quality owing to their position, their nature, and their climate, as is easily seen in pa.s.sing over different places distinguished by special features; hence we see a cause of variation for the animals and plants which live in these different places. But what we do not sufficiently know, and even what we generally refuse to believe, is that each place itself changes with time in exposure, in climate, in nature, and quality, although with a slowness so great in relation to our own continuance that we attribute to it a perfect stability.

"Now, in either case, these changed localities proportionally change the circ.u.mstances relative to the organisms which inhabit them, and the latter then give rise to other influences bearing on these same beings.

"We perceive from this that, if there are extremes in these changes, there are also gradations--namely, degrees which are intermediate and which fill the interval. Consequently there are also gradations in the differences which distinguish what we call _species_.

"It is then evident that the whole surface of the earth offers, in the nature and situation of the matters which occupy its different points, a diversity of circ.u.mstances which is throughout in relation with that of the forms and parts of animals, independent of the special diversity which necessarily results from the progress of the composition of organization in each animal.

"In each locality where animals can live, the circ.u.mstances which establish there an order of things remain for a long time the same, and really change there only with a slowness so great that man cannot directly notice them. He is obliged to consult monuments to recognize that in each one of these places the order of things that he discovers there has not always been the same, and to perceive that it will change more.

"The races of animals which live in each of these places should, then, retain their customary habits there also for a long time; hence to us seems an apparent constancy of races which we call _species_--constancy which has originated among us the idea that these races are as ancient as nature.

"But in the different points of the earth's surface which can be inhabited, nature and the situation of the places and climates const.i.tute there, for the animals as for the plants, _different circ.u.mstances_ of all sorts of degrees. The animals which inhabit these different places should then differ from each other, not only on account of the state of nature of the organization in each race, but, besides, by reason of the habits that the individuals of each race there are forced to have; so, in proportion as he traverses the larger parts of the earth's surface the observing naturalist sees circ.u.mstances changing in a manner somewhat noticeable; he constantly sees that the species change proportionately in their characters.

"Now, the true order of things necessary to consider in all this consists in recognizing:

"1. That every slight change maintained under the circ.u.mstances where occur each race of animals, brings about in them a real change in their wants.

"2. That every change in the wants of animals necessitates in them other movements (_actions_) to satisfy the new needs, and consequently other habits.

"3. That every new want necessitating new actions to satisfy it, demands of the animal which feels it both the more frequent use of such of its parts of which before it made less use, which develops and considerably enlarges them, and the use of new parts which necessity has caused to insensibly develop in it by the effects of its inner feelings; which I shall constantly prove by known facts.

"Thus, to arrive at a knowledge of the true causes of so many different forms and so many different habits of which the known animals offer us examples, it is necessary to consider that circ.u.mstances infinitely diversified, but all slowly changing, into which the animals of each race are successively thrown, have caused, for each of them, new wants and necessarily changes in their habits.

Moreover, this truth, which cannot be denied, being once recognized, it will be easy to see how the new needs have been able to be satisfied, and the new habits formed, if any attention be given to the two following laws of nature, which observation always confirms:

"_First Law._

"In every animal which has not exceeded the term of its development, the more frequent and sustained use of any organ gradually strengthens this organ, develops and enlarges it, and gives it a strength proportioned to the length of time of such use; while the constant lack of use of such an organ imperceptibly weakens it, causes it to become reduced, progressively diminishes its faculties, and ends in its disappearance.

"_Second Law._

"Everything which nature has caused individuals to acquire or lose by the influence of the circ.u.mstances to which their race may be for a long time exposed, and consequently by the influence of the predominant use of such an organ, or by that of the constant lack of use of such part, it preserves by heredity (_generation_) and pa.s.ses on to the new individuals which descend from it, provided that the changes thus acquired are common to both s.e.xes, or to those which have given origin to these new individuals.

"These are the two fundamental truths which can be misunderstood only by those who have never observed or followed nature in its operations, or only by those who allow themselves to fall into the error which I have combated.

"Naturalists having observed that the forms of the parts of animals compared with the uses of these parts are always in perfect accord, have thought that the forms and conditions of parts have caused the function; but this is a mistake, for it is easy to demonstrate by observation that it is, on the contrary, the needs and uses of organs which have developed these same parts, which have even given origin to them where they did not exist, and which consequently have given rise to the condition in which we observe them in each animal.

"If this were not so, it would have been necessary for nature to have created for the parts of animals as many forms as the diversity of circ.u.mstances in which they have to live had required, and that these forms and also the circ.u.mstances had never varied.

"This is certainly not the existing order of things, and if it were really such, we should not have the race-horses of England; we should not have our great draft horses, so clumsy and so different from the first named, for nature herself has not produced their like; we should not, for the same reason, have terrier dogs with bow legs, greyhounds so swift in running, water-spaniels, etc.; we should not have tailless fowls, fantail pigeons, etc.; finally, we could cultivate the wild plants as much as we pleased in the rich and fertile soil of our gardens without fearing to see them change by long culture.

"For a long time we have felt the force of the saying which has pa.s.sed into the well-known proverb--_habits form a second nature_.

"a.s.suredly, if the habits and nature of each animal can never vary, the proverb is false, has no foundation, and does not apply to the instances which led to its being spoken.

"If we should seriously consider all that I have just stated, it might be thought that I had good reason when in my work ent.i.tled _Recherches sur les Corps vivans_ (p. 50) I established the following proposition:

"'It is not the organs--that is to say, the nature and form of the parts of the body of an animal--which have given rise to its habits and its special faculties; but it is, on the contrary, its habits, its manner of life, and the circ.u.mstances in which are placed the individuals from which it originates, which have, with time, brought about the form of its body, the number and condition of its organs, finally, the faculties which it enjoys.'

"If we weigh this proposition, and if we recall all the observations which nature and the state of things continually lead us to do, then its importance and its solidity will become more evident.

"Time and favorable circ.u.mstances are, as I have already said, the two princ.i.p.al means which nature employs to give existence to all her productions: we know that time for her has no limits, and that consequently it is ever at her disposal.

"As to the circ.u.mstances of which she has need, and which she uses still daily to cause variations in all that she continues to produce, we can say that they are, in some degree, for her inexhaustible.

"The princ.i.p.al circ.u.mstances arise from the influence of climate; from those of different temperatures of the atmosphere, and from all the environing media; from that of the diversity of different localities and their situation; from that of habits, the ordinary movements, the most frequent actions; finally, from that of means of preservation, of mode of living, of defence, of reproduction, etc.

"Moreover, owing to these diverse influences, the faculties increase and become stronger by use, become differentiated by the new habits preserved for long ages, and insensibly the organization, the consistence--in a word, the nature and condition of parts, as also of the organs--partic.i.p.ate in the results of all these influences, become preserved, and are propagated by generation.

"These truths, which are only the results of the two natural laws above stated, are in every case completely confirmed by facts; they clearly indicate the course of nature in all the diversity of its products.

"But instead of contenting ourselves with generalities which might be considered as hypothetical, let us directly examine the facts, and consider, in the animals, the result of the use or disuse of their organs on the organs themselves, according to the habits that each race has been compelled to contract.

"I shall now attempt to prove that the constant lack of exercise of organs at first diminishes their faculties, gradually impoverishes them, and ends by making them disappear, or even causing them to be atrophied, if this lack of use is perpetuated for a very long time through successive generations of animals of the same race.

"I shall next prove that, on the contrary, the habit of exercising an organ, in every animal which has not attained the limit of the diminution of its faculties, not only perfects and increases the faculties of this organ, but, besides, enables it to acquire developments and dimensions which insensibly change it; so that with time it renders it very different from the same organ in another animal which exercises it much less.

"_The lack of use of an organ, become constant by the habits formed, gradually impoverishes this organ, and ends by causing it to disappear and even to destroy it._

"As such a proposition can only be admitted on proof, and not by its simple announcement, let us prove it by the citation of the leading known facts on which it is based.

"The vertebrate animals, whose plan of organization is in all nearly the same, although they offer much diversity in their parts, have jaws armed with _teeth_; moreover, those among them which circ.u.mstances have placed in the habit of swallowing their food without previous _mastication_ are exposed to the result that their teeth become undeveloped. These teeth, then, either remain concealed between the bony edges of the jaws, without appearing above, or even their gums are found to have been atrophied.

"In the baleen whales, which have been supposed to be completely deprived of teeth, M. Geoffroy has found them concealed in the jaws of the _foetus_ of this animal. This professor has also found in the birds the groove where the teeth should be situated; but they are no longer to be seen there.

"In the cla.s.s even of mammals, which comprises the most perfect animals, and chiefly those in which the vertebrate plan of organization is most perfectly carried out, not only the baleen has no usable teeth, but the ant-eater (_Myrmecophaga_) is also in the same condition, whose habit of not masticating its food has been for a long time established and preserved in its race.

"The presence of eyes in the head is a characteristic of a great number of different animals, and becomes an essential part of the plan of organization of vertebrates.

"Nevertheless the mole, which owing to its habits makes very little use of vision, has only very small eyes, which are scarcely visible, since they exercise these organs to a very slight extent.

"The _Aspalax_ of Olivier (_Voyage en Egypte et en Perse_, ii.

pl. 28 f. 2), which lives under ground like the mole, and which probably exposes itself still less than that animal to the light of day, has totally lost the power of sight; also it possesses only vestiges of the organ of which it is the seat; and yet these vestiges are wholly concealed under the skin and other parts which cover them, and do not permit the least access to the light.

"The _Proteus_, an aquatic reptile allied to the salamander in its structure, and which lives in the dark subterranean waters of deep caves, has, like the _Aspalax_, only vestiges of the organs of sight--vestiges which are covered and concealed in the same manner.

"We turn to a decisive consideration relative to this question.

"Light does not penetrate everywhere; consequently animals which habitually live in situations where it does not penetrate lack the occasion of exercising the organs of sight, if nature has provided them with them. Moreover, the animals which make part of the plan of organization in which _eyes_ are necessarily present, have originally had them. However, since we find them among those which are deprived of the use of this organ, and which have only vestiges concealed and covered over, it should be evident that the impoverishment and even the disappearance of these organs are the result of a constant lack of exercise.