This leads him to consider what is life, and he remarks (p. xv.) that it does not exist without external stimuli. The conditions necessary for the existence of life are found completely developed in the simplest organization. We are then led to inquire how this organization, by reason of certain changes, can give rise to other organisms less simple, and finally originate creatures becoming gradually more complicated, as we see in ascending the animal scale. Then employing the two following considerations, he believes he perceives the solution of the problem which has occupied his thoughts.
He then cites as factors (1) use and disuse; (2) the movement of internal fluids by which passages are opened through the cellular tissue in which they move, and finally create different organs. Hence the movement of fluids in the interior of animals, and the influence of new circumstances as animals gradually expose themselves to them in spreading into every inhabitable place, are the two general causes which have produced the different animals in the condition we now see them. Meanwhile he perceived the importance of the preservation by heredity, though he nowhere uses that word, in the new individuals reproduced of everything which the results of the life and influencing circumstances had caused to be acquired in the organization of those which have transmitted existence to them.
In the Discours préliminaire, referring to the progression in organization of animals from the simplest to man, as also to the successive acquisition of different special organs, and consequently of as many faculties as new organs obtained, he remarks:
“Then we can perceive how needs (besoins), at the outset reduced to nullity, and of which the number gradually increases, have produced the inclination (penchant) to actions fitted to satisfy it; how the actions, becoming habitual and energetic, have caused the development of the organs which execute them; how the force which excites the organic movements may, in the simplest animals, be outside of them and yet animate them; how, then, this force has been transported and fixed in the animal itself; finally, how it then has become the source of sensibility, and in the end that of acts of intelligence.
“I shall add that if this method had been followed, then sensation would not have been regarded as the general and immediate cause of organic movements, and it would not have been said that life is a series of movements which are executed in virtue of sensations received by different organs; or, in other words, that all the vital movements are the product of impressions received by the sensitive parts.[179]
“This cause seems, up to a certain point, established as regards the most perfect animals; but had it been so relatively to all living beings, they should all be endowed with the power of sensation. But it cannot be proved that this is the case with plants, and it cannot likewise be proved that it is so with all the animals known.
“But nature in creating her organisms has not begun by suddenly establishing a faculty so eminent as that of sensation: she has had the means of producing this faculty in the imperfect animals of the first classes of the animal kingdom,” referring to the Protozoa. But she has accomplished this gradually and successively. “Nature has progressively created the different special organs, also the faculties which animals enjoy.”
He remarks that though it is indispensable to classify living forms, yet that our classifications are all artificial; that species, genera, families, orders, and classes do not exist in nature—only the individuals really exist. In the third chapter he gives the old definition of species, that they are fixed and immutable, and then speaks of the animal series, saying:
“I do not mean by this to say that the existing animals form a very simple series, and especially evenly graduated; but I claim that they form a branched series,[180] irregularly graduated, and which has no discontinuity in its parts, or which, at least, has not always had, if it is true that, owing to the extinction of some species, there are some breaks. It follows that the species which terminates each branch of the general series is connected at least on one side with other species which intergrade with it” (p. 59).
He then points out the difficulty of determining what are species in certain large genera, such as Papilio, Ichneumon, etc. How new species arise is shown by observation.
“A number of facts teaches us that in proportion as the individuals of one of our species are subjected to changes in situation, climate, mode of life or habits, they thereby receive influences which gradually change the consistence and the proportions of their parts, their form, their faculties, even their structure; so that it follows that all of them after a time participate in the changes to which they have been subjected.
“In the same climate very different situations and exposures cause simple variations in the individuals occurring there; but, after the lapse of time, the continual differences of situation of the individuals of which I speak, which live and successively reproduce under the same circumstances, produce differences in them which become, in some degree, essential to their existence, so that at the end of many successive generations these individuals, which originally belonged to another species, became finally transformed into a new species distinct from the other.
“For example, should the seeds of a grass or of any other plant natural to a moist field be carried by any means at first to the slope of a neighboring hill, where the soil, although more elevated, will yet be sufficiently moist to allow the plant to live there, and if it results, after having lived there and having passed through several generations, that it gradually reaches the dry and almost arid soil of a mountain side; if the plant succeeds in living there, and perpetuates itself there during a series of generations, it will then be so changed that any botanists who should find it there would make a distinct species of it.
“The same thing happens in the case of animals which circumstances have forced to change in climate, mode of life, and habits; but in their case the influences of the causes which I have just cited need still more time than the plants to bring about notable changes in the individuals.
“The idea of embracing, under the name of species, a collection of like individuals which are perpetuated by generation, and which have remained the same as long as nature has endured, implies the necessity that the individuals of one and the same species should not cross with individuals of a different species.
“Unfortunately observation has proved, and still proves every day, that this consideration is unfounded; for hybrids, very common among plants, and the pairings which we often observe between the individuals of very different species of animals, have led us to see that the limits between these supposed constant species are not so fixed as has been imagined.
“In truth, nothing often results from these singular unions, especially if they are very ill-assorted, and then the individuals which do result from them are usually infertile; but also, when the disparities are less great, we know that the default in question does not occur.
“But this cause only suffices to create, step by step, varieties which finally become races, and which, with time, constitute what we call species.
“To decide whether the idea which is formed of the species has any real foundation, let us return to the considerations which I have already explained; they lead us to see:
“1. That all the organized bodies of our globe are true productions of Nature, which she has successively formed after the lapse of much time;
“2. That, in her course. Nature has begun, and begins over again every day, to form the simplest organisms, and that she directly creates only those, namely, which are the first germs (ébauches) of organization, which are designated by the expression of spontaneous generations;
“3. That the first germs of the animal and plant having been formed in appropriate places and circumstances, the faculties of a beginning life and of an organic movement established, have necessarily gradually developed the organs, and that with time they have diversified them, as also the parts;
“4. That the power of growth in each part of the organized body being inherent in the first created forms of life, it has given rise to different modes of multiplication and of regeneration of individuals; and that consequently the progress acquired in the composition of the organization and in the shape and diversity of the parts has been preserved;
“5. That with the aid of sufficient time, of circumstances which have been necessarily favorable, of changes of condition that every part of the earth’s surface has successively undergone—in a word, by the power which new situations and new habits have of modifying the organs of living beings, all those which now exist have been gradually formed such as we now see them;
“6. Finally, that, according to a similar order of things, living beings having undergone each of the more or less great changes in the condition of their structure and parts, that which we call a species among them has been gradually and successively so formed, having only a relative constancy in its condition, and not being as old as Nature herself.
“But, it will be said, when it is supposed that by the aid of much time and of an infinite variation in circumstances, Nature has gradually formed the different animals known to us, shall we not be stopped in this supposition by the simple consideration of the admirable diversity which we observe in the instincts of different animals, and by that of the marvels of every kind presented by their different kinds of industry?
“Shall we dare to extend the spirit of system so far as to say that it is Nature who has herself alone created this astonishing diversity of means, of contrivances, of skill, of precautions, of patience, of which the industry of animals offers us so many examples? What we observe in this respect in the simple class of insects, is it not a thousand times more than sufficient to make us realize that the limit to the power of Nature in nowise permits her to herself produce so many marvels, but to force the most obstinate philosopher to recognize that here the will of the Supreme Author of all things has been necessary, and has alone sufficed to create so many admirable things?
“Without doubt, one would be rash or, rather, wholly insensate, to pretend to assign limits to the power of the first Author of all things; but, aside from that, no one could dare to say that this infinite power could not will that which Nature even shows us it has willed”[181] (p. 67).
Referring to the alleged proof of the fixity of species brought forward by Cuvier in the Annales du Muséum d’Histoire naturelle (i., pp. 235 and 236) that the mummied birds, crocodiles, and other animals of Egypt present no differences from those now living, Lamarck says:
“It would assuredly be very singular if it were otherwise, because the position of Egypt and its climate are still almost exactly what they were at that epoch. Moreover, the birds which live there still exist under the same circumstances as they were then, not having been obliged to change their habits.
“Moreover, who does not perceive that birds, which can so easily change their situation and seek places which suit them are less subject than many other animals to the variations of local circumstances, and hence less restricted in their habits.”