Form and Function: A Contribution to the History of Animal Morphology

by E. S. Russell

According to these "axioms" function is a direct and mechanical effect of structure.

The curious thing is that in spite of his avowed materialism, Lamarck's conception of life and evolution is profoundly psychological, and from the conflict of his materialism and his vitalism (of which he was himself hardly conscious), arise most of the obscurities and the irreductible self-contradiction of his theory.

Lamarck divided animals (psychologically!) into three great groups—apathetic or insensitive animals, animals endowed with sensation, and intelligent animals. The first group, which comprise all the lower Invertebrates, are distinguished from other animals by the fact that their actions are directly and mechanically due to the excitations of the environment; they have no principle of reaction to external influences, but passively prolong into action the excitations they receive from without. They are irritable merely. The second group are distinguished from the first by their possessing, in addition to irritability, a power which Lamarck calls the sentiment intérieur. He has some difficulty in defining exactly what he means by it:—"I have no term to express this internal power possessed not only by intelligent animals but also by those that are endowed merely with the faculty of sensation; it is a power which, when set in action by the feeling of a need, causes the individual to act at once, i.e., in the very moment of the sensation it experiences; and if the individual is of those that are endowed with intelligence it nevertheless acts in such a case entirely without premeditation and before any mental operation has brought its will into play" (p. 24).

It is the power we call instinct in animals (p. 25), and it implies neither consciousness nor will. It acts by transforming external into internal excitations.

To this second group of animals, possessing the sentiment intérieur, belong the higher Invertebrates, notably insects and molluscs. Only animals possessed of a more or less centralised nervous system can manifest this sentiment, or principle of (unconscious) reaction to external stimuli.

The higher animals, or the four Vertebrate classes, form the group of "intelligent animals." In virtue of their more complex organisation they possess in addition to the sentiment intérieur the faculties of intelligence and will.

Now, broadly put, Lamarck's theory of evolution is that new organs are formed in direct reaction to needs (besoins) experienced by the sentiment intérieur. The sentiment intérieur is therefore the cause not only of instinctive action but also of all morphogenetic processes. Will and intelligence (which are confined to a relatively small number of animals) have little or nothing to do directly with evolution.

To understand the working-out of Lamarck's evolution-theory we must revert to his conception of the Échelle des êtres. What he wrote in the Philosophie zoologique is here repeated in the work of 1816 with little modification.

There is a real progression from the simpler to the more complex organisations; Nature has gradually complicated her creatures by giving them new organs and therefore new faculties.

It is interesting to note that Lamarck expressly refers to Bonnet (p. 110), but refuses to accept his view of an Échelle 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 Échelle 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 assume that Vertebrates and Invertebrates alike are formed upon one common plan of structure.