110), but refuses to accept his view of an _ech.e.l.le_ extending down into the inorganic. Like Bonnet, however, and like the German transcendentalists, Lamarck makes man the goal of evolution (p. 116). He makes it quite clear that his _ech.e.l.le_ is a functional one, for he links Vertebrates to molluscs even while expressly admitting that they are not connected by any structural intermediates (p. 123). He does not fall into the error of the transcendentalists and a.s.sume that Vertebrates and Invertebrates alike are formed upon one common plan of structure.
The progression of organisation shown by the animal kingdom has not been altogether regular and uninterrupted:--"The progression in complexity of organisation shows here and there, in the general animal series, anomalies induced by the influence of environment and by the influence of the habits contracted" (_Phil. zool._, i., p. 145).
There are thus really two causes at work to produce the variety of organisation as it appears to us, one which tends to produce a regular increase in complexity, and one which disturbs and diversifies this regular advance.
The first cause Lamarck calls the vital power (_pouvoir de la vie_); the other may be called the influence of circ.u.mstance (_Anim. s. Vert._, p.
134). To the latter cause are due the lacunae, the blind alleys, and the complications which the otherwise simple scale of perfection shows.
To explain both these aspects of evolution Lamarck propounded in his volume of 1816 four laws, which read as follows:--
"_First Law_.--Life, by its own forces, tends continually to increase the volume of every body possessing it, and to extend the dimensions of its parts, up to a limit which it brings about itself.
"_Second Law_.--The production of a new organ in an animal body results from the arisal and continuance of a new need, and from the new movement which this need brings into being and sustains.
"_Third Law_.--The degree of development of organs and their force of action are always proportionate to the use made of these organs.
"_Fourth Law_.--All that has been acquired, imprinted or changed in the organisation of the individual during the course of its life is preserved by generation and transmitted to the new individuals that descend from the individual so modified" (pp. 151-2).
It is mainly but not entirely by reason of the first of these laws that organisation tends to progress, and mainly by reason of the second and third that difference of environment brings about diversity of organisation. In virtue of the fourth law the acquirements of the individual become the property of the race.
Lamarck"s exposition of his first law, that life tends by its own powers to enlarge and extend its bodily instrument, is vague and difficult to understand. He has already explained some pages back how the first organisms arose by spontaneous generation in the form of minute gelatinous utricles (_cf._ Oken). He conceives that it is in the movements of the fluids proper to the organism that the power resides to enlarge and extend the body. Nutrition alone is not sufficient to bring about extension; a special force is required, acting from within outwards (p. 153). In the most primitive organisms the movements of the vital fluids are weak and slow, but in the course of evolution they gradually accelerate, and, becoming more rapid, trace out ca.n.a.ls in the delicate tissue which contains them, and finally form organs.
Subtle fluids play a great part in Lamarck"s biology: they take the place of the soul or entelechy which the vitalists would postulate to explain organic happenings. Lamarck seems in this to follow certain of the old materialists, who conceived the soul to be formed of a matter more subtle than the ordinary.[342]
In his second law Lamarck"s essentially vitalistic att.i.tude comes out very clearly, for it states that a psychological moment enters into all new production of form, that the ultimate cause of the development of new form is the need felt by the organism. This need is of course not a conscious one, it is a need perceived by the _sentiment interieur_.
In the large group of apathetic or insensitive animals, which do not possess this faculty, needs cannot be experienced; accordingly new organs are here formed directly and mechanically, by the movements of the vital fluids set in action by excitations from without--the evolution, like the behaviour, of these animals is due to the direct and physical action of the environment. "But this is not the case with the more highly organised animals which possess _feeling_. They experience needs, and each need felt, acting upon their "inner feeling,"
immediately directs the fluids and the forces to the part of the body where action can satisfy the need. Now, if there exists at this point an organ capable of performing the required action, it is quickly stimulated to act; and if the organ does not exist and the need is pressing and sustained, bit by bit the organ is produced and developed in proportion to the continuity and the energy of its use" (p. 155).
In intelligent animals the _sentiment interieur_ may be moved by thought or will.
As an example of the way in which the law works Lamarck takes the hypothetical case of a gastropod mollusc, which as it creeps along experiences dimly the need to feel the objects in front of it. It makes an effort (unconscious, be it noted) to touch these objects with the anterior portions of its head, and sends forward continually to these parts a great volume of nervous and other fluids. From these efforts and the repeated afflux of fluids there must result a development of the nerves supplying these parts. And as, along with the nervous fluids, nutritive juices constantly flow to the parts, there must result the formation of two or four tentacles in the places to which these fluids are directed. A curious mixture of mechanistic "explanations" and vitalistic hypothesis!
In his third law, that use and disuse are powerful to modify organs, Lamarck is upon more solid ground, and can point to many instances of the visible effect of these factors of change. It is of course rather closely bound up with his second law and may even be regarded as an extension of it.
The law has reference to one of the most powerful means employed by Nature to diversify species, a means which comes into play whenever the environment changes. The cause of the great diversity shown by animal species is indeed ultimately to be sought in the environment. As the imperfect and earliest forms developed they spread over the earth and invaded the utmost corners of it:--"One can imagine what an enormous variety of habitats, stations, climates, available foods, environing media, etc., animals and plants have had to endure, as the existing species were forced to change their place of abode. And although these changes have taken place with extreme slowness ... their reality, necessitated by various causes, has none the less induced the species affected by them slowly to change their manner of life and their habitual actions. Through the effects of the second and third of the laws cited above, these induced activity-changes must have brought into being new organs, and must have been able to develop them further if more frequent use was made of them; they must in the same way have been capable of bringing about the degeneration and finally the complete disappearance of existing organs which had become useless" (p. 161).
On the other hand, if the environment does not change, species remain constant.
It is to be noted that change in environment is rather the occasion than the cause of modification; the environment induces the organism to change its habitual way of life; it sets up new needs, to satisfy which the organism must modify its structure. It is the organism that takes the active part in all this, the action of the environment is indirect.
Of Lamarck"s fourth law, which a.s.serts the transmission of acquired characters, little need here be said in the way of exposition. Upon the truth of it depends of course Lamarck"s whole theory. He himself never dreamed that anyone would ever dispute it.
Lamarck sums up as follows:--"By the four laws which I have just enunciated all the facts of organisation seem to me to be easily explained; the progression in the complexity of organisation of animals, and in their faculties, seems to me easy to conceive; so, too, the means which Nature has employed to diversify animals, and bring them to the state in which we now see them, become easily determinable" (p. 168).
It is never made quite clear, we may note in pa.s.sing, how far his second and third laws tend to bring about an increase in complexity, in addition to diversifying animals.[343]
"The function creates the organ," this would seem to be the kernel of Lamarck"s doctrine. But how does he reconcile this essentially vitalistic conception with his strictly materialistic philosophy?
We have seen that irritability, the _sentiment interieur_, and intelligence itself, are the effects of organisation. We are told farther on that both the _sentiment_ and intelligence are caused by nervous fluids. A great part of both the _Philosophie zoologique_ and the introduction to the _Animaux sans Vertebres_ is given up to the exposition of a materialistic psychology of animals and man, based entirely upon this hypothesis of nervous fluids. Thus habits are due to the fluids hollowing out definite paths for themselves.
The _sentiment interieur_ acts by directing the movements of the subtle fluids of the body (which are themselves modifications of the nervous fluids) upon the parts where a new organ is needed. But if it is itself only a result of the movement of nervous fluids? Again, how can a need be "felt" by a nervous fluid? This is an entirely psychological notion and cannot be applied to a purely material system. Whence arises the power of the _sentiment interieur_ to ca.n.a.lise the energies of the organism, so to direct and co-ordinate them that they build up purposive structures, or effect purposive actions (as in all instinctive behaviour)? Either the _sentiment interieur_ is a psychological faculty, or it is nothing.
There is no doubt that, as expressed by Lamarck, the conception conceals a radical confusion of thought. It is not possible to be a thorough-going materialist, and at the same time to believe that new organs are formed in direct response to needs felt by the organism.
Lamarck could never resolve this antinomy, and his speculations were thrown into confusion by it. To this cause is due the frequent obscurity of his writings.
Should we be right in laying stress upon the psychological side of Lamarck"s theory, and disregarding the materialistic dress in which, perhaps under the influence of the materialism current in his youth, he clothed his essentially vitalistic thought? Everything goes to prove it--his constant preoccupation with psychological questions, his tacit a.s.similation of organ-formation to instinctive behaviour, his constant insistence on the importance of _besoin_ and _habitude_.
Let us not forget the profundity of his main idea, that, exception made for the lower forms, the animal is essentially active, that it always _reacts_ to the external world, is never pa.s.sively acted upon. Let us not forget that he pointed out the essentially psychological moment implied in all processes of individual adaptation. With keen insight he realised that conscious intelligence counts for little in evolution, and focussed attention upon the unconscious but obscurely psychical processes of instinct and morphogenesis.
Not without reason have the later schools of evolutionary thought, who developed the psychological and vitalistic side of his doctrine, called themselves Neo-Lamarckians.
We shall say then that Lamarck, in spite of his materialism, was the founder of the "psychological" theory of evolution.
Lamarck stood curiously aloof and apart from the scientific thought of his day.[344] He took no interest in the morphological problems that filled the minds of Cuvier and Geoffroy; he had indeed no feeling at all for morphology. He did not realise, like Cuvier, the _convenance des parties_, the marvellous co-ordination of parts to form a whole; he had little conception of what is really implied in the word "organism." He was not, like Geoffroy, imbued with a lively sense of the unity of plan and composition, and of the significance of vestigial organs as witnesses to that unity. He seems not to have known of the recapitulation theory, of which he might have made such good use as powerful evidence for evolution. Even with the German transcendentalists, with whom in the looseness of his generalisations he shows some affinity, he seems not to have been specially acquainted.
He was interested more in the problems suggested to him by his daily work in the museum. He wanted to know why species graded so annoyingly into one another; he wanted to examine critically his haunting suspicion that species were really not distinct, and that cla.s.sification was purely conventional. The question, too, of the adaptation of species to their environment, the problem of ecological adaptation, in distinction to that of functional adaptation which interested Cuvier so greatly, came vividly before him as he worked through the vast collections of the museum. He was the first systematist to occupy himself in a philosophical manner with the problems of general biology. He introduced new problems and a new way of looking at old. With Lamarck the problem of species and the problem of ecological adaptation enter into general biology.
The one point in which he does definitely carry on the thought of his predecessors is his conception of the animal kingdom as forming a scale of (functional) perfection. He did not go to the same extreme as Bonnet; he did not even consider that the animal series was a continuation of the vegetable series; in his opinion they formed two diverging scales.
He recognised, too, that among animals there was no simple and regular gradation from the lowest to the highest, but that the orderly progression was disturbed and diverted by the necessity of adaptation to different environments. It is interesting to note that in developing this idea he arrived at a roughly accurate distinction between h.o.m.ologous and a.n.a.logous structures. More importance, he thought, was to be attributed in cla.s.sifying animals to characters which appeared due to the "plan of Nature" than to such as were produced by an external modifying cause (p. 299). But he did not formulate the distinction in any strictly morphological way.
As his ideas developed he laid less stress upon the simplicity and continuity of the scale; in his supplementary remarks to the Introduction of 1816 he admits that the series is really very much branched, and even that there may be two distinct series among animals instead of one. His last schema of the course of evolution shows no little a.n.a.logy with the genealogical trees of Darwinian speculation. It is headed "The presumed _Order_ of the formation of Animals, showing two separate partly-branching series," and it reads as follows:--
I.--_Series of Non-articulated_ II.--_Series of Articulated_ _Animals_. _Animals_.
I -- Infusoria.
n s A Polyps.
e n n i ---------------- s m i a Ascidians. Radiates. Worms.
t l i s -------------- v .
e Epizoa.
" -- S A Acephala. Annelids. Insects.
e n n i s m Molluscs. ------------- i a t l Arachnids.
i s Crustacea.
v .
e " -- Cirripedes.
I n -- t A e n Fishes.
l i Reptiles.
l m Birds.
i a Mammals.
g l e s -- n .
t
It is interesting to note that Vertebrates are placed between the two series, and are now not linked on directly to any Invertebrate group.
Lamarck"s theory had little success. There is evidence, however, that both Meckel and Geoffroy owed a good many of their evolutionary ideas to Lamarck, and Cuvier paid him at least the compliment of criticising his theory,[345] not distinguishing it, however, very clearly from the evolutionary theories of the transcendentalists. But, speaking generally, Lamarck"s theory of evolution exercised very little influence upon his contemporaries. This was probably due partly to the obscurity and confusion of his thought, partly to his lack of sympathy with the biological thought of his day, which was preponderatingly morphological.