THE "PHILOSOPHIE ZOOLOGIQUE"
Lamarck"s mature views on the theory of descent comprise a portion of his celebrated _Philosophie zoologique_. We will let him tell the story of creation by natural causes so far as possible in his own words.
In the _avertiss.e.m.e.nt_, or preface, he says that his experience has led him to realize that a body of precepts and of principles relating to the study of animals and even applicable to other parts of the natural sciences would now be useful, our knowledge of zoological facts having, for about thirty years, made considerable progress.
After referring to the differences in structure and faculties characterizing animals of different groups, he proceeds to outline his theory, and begins by asking:
"How, indeed, can I consider the singular modification in the structure of animals, as we glance over the series from the most perfect to the least perfect, without asking how we can account for a fact so positive and so remarkable--a fact attested to me by so many proofs? Should I not think that nature has successively produced the different living beings by proceeding from the most simple to the most compound; because in ascending the animal scale from the most imperfect up to the most perfect, the organization perfects itself and becomes gradually complicated in a most remarkable way?"
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 pa.s.sages 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 circ.u.mstances_ 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 circ.u.mstances had caused to be acquired in the organization of those which have transmitted existence to them.
In the _Discours preliminaire_, 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 cla.s.ses 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 cla.s.sify living forms, yet that our cla.s.sifications are all artificial; that species, genera, families, orders, and cla.s.ses 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 partic.i.p.ate 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 circ.u.mstances, 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 gra.s.s 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 pa.s.sed 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 circ.u.mstances 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-a.s.sorted, 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, const.i.tute 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 (_ebauches_) 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 circ.u.mstances, 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 circ.u.mstances 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 circ.u.mstances, 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 cla.s.s 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 a.s.sign 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 Museum 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 a.s.suredly 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 circ.u.mstances 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 circ.u.mstances, and hence less restricted in their habits."
He adds the fact that the animals in question have inhabited Egypt for two or three thousand years, and not necessarily from all time, and that this is not time enough for marked changes. He then gives the following definition of species, which is the best ever offered: "Species, then, have only a relative stability, and are invariable only temporarily."
"Yet, to facilitate the study and knowledge of so many different organisms it is useful to give the name of _species_ to every similar collection of similar individuals which are perpetuated by heredity (_generation_) in the same condition, so long as the circ.u.mstances of their situation do not change enough to render variable their habits, character, and form."
He then discusses fossil species in the way already described in Chapter III. (p. 75).
The subject of the checks upon over-population by the smaller and weaker animals, or the struggle for existence, is thus discussed in Chapter IV.:
"Owing to the extreme multiplication of the small species, and especially of the most imperfect animals, the multiplicity of individuals might be prejudicial to the preservation of the species, to that of the progress acquired in the improvement of the organization--in a word, to the general order, if nature had not taken precautions to keep this multiplication within due limits over which she would never pa.s.s.
"Animals devour one another, except those which live only on plants; but the latter are exposed to being devoured by the carnivorous animals.
"We know that it is the strongest and the best armed which devour the weaker, and that the larger kinds devour the smaller.
Nevertheless, the individuals of a single species rarely devour each other: they war upon other races.[182]
"The multiplication of the small species of animals is so considerable, and the renewals of their generations are so prompt, that these small species would render the earth uninhabitable to the others if nature had not set a limit to their prodigious multiplication. But since they serve as prey for a mult.i.tude of other animals, as the length of their life is very limited, and as the lowering of the temperature kills them, their numbers are always maintained in proper proportions for the preservation of their races and that of others.
"As to the larger and stronger animals, they would be too dominant and injure the preservation of other races if they should multiply in too great proportions. But their races devouring each other, they would only multiply slowly and in a small number at a time; this would maintain in this respect the kind of equilibrium which should exist.
"Finally, only man, considered separately from all which is characteristic of him, seems capable of multiplying indefinitely, because his intelligence and his resources secure him from seeing his increase arrested by the voracity of any animals. He exercises over them such a supremacy that, instead of fearing the larger and stronger races of animals, he is thus rather capable of destroying them, and he continually checks their increase.
"But nature has given him numerous pa.s.sions, which, unfortunately, developing with his intelligence, thus place a great obstacle to the extreme multiplication of the individuals of his species.
"Indeed, it seems as if man had taken it upon himself unceasingly to reduce the number of his fellow-creatures; for never, I do not hesitate to say, will the earth be covered with the population that it could maintain. Several of its habitable parts would always be alternately very spa.r.s.ely populated, although the time for these alternate changes would be to us measureless.
"Thus by these wise precautions everything is preserved in the established order; the changes and perpetual renewals which are observable in this order are maintained within limits over which they cannot pa.s.s; the races of living beings all subsist in spite of their variations; the progress acquired in the improvement of the organization is not lost; everything which appears to be disordered, overturned, anomalous, reenters unceasingly into the general order, and even cooperates with it; and especially and always the will of the sublime Author of nature and of all existing things is invariably executed" (pp. 98-101).