This division, inspired by the neurology of the period and by the evolution of the species, remains pertinent for some present-day authors.20 In Lamarck’s conception, there is no relationship between a zone and a mental function,21 but there are more complex forms of the organization of the neurological activity that open into various modes of treating information that are experienced as sensations (in the brainstem), as feelings (in the limbic system), or as thinking (in the neocortex). I find it important to insist on the fact that intelligence is not situated in the neocortex, but that the neocortex allows for a more complex organization22 of the entire neurological activity that unfolds into the capacity to think. This explains that the intelligence of mammals is more or less developed according to the advanced degree of the organization permitted by the neocortex of a species.23 The notion that each structure of the brain permits the creation of more complex organizations of the whole nervous system already announces some of the present neurological models.24 An essential point is that consciousness requires a simultaneous coordination of many parts of the nervous system, but not necessarily of the entire system.
The relevance of this division is justified by the fact that these five parts of the nervous system correspond to its evolution and to the development of mental capacities. There exist animals with a neocortex (down to the rat), others without a neocortex, others without a brainstem, some without a spinal cord, and finally some that do not even have isolated sensorimotor centers. However the neurological hierarchy is not as strict as Lamarck and Darwinians of the first half of the century had assumed. For example, the highly intelligent and skillful octopus has a nervous system that cannot easily be fitted in the classical evolutionary frame.
This model was taken up in the 1950s by MacLean. His formulation, presented in Darwinian language, became fashionable with psysâ (see the Glossary) and philosophers.25 However useful it might be, when used as a metaphor or a shortcut, this model does not take into account the complexity of the connections in the brain.26 From Propensity to Inclination27
Instinct is, in all sensitive beings, the production of an inner sentiment which it possesses, a very obscure sentiment which, in certain circumstances, leads it to execute actions without knowing it, without previous cause, and without the use of an idea, and then, without the participation of will. (Lamarck, 1820, Systeme Analytique, II.II.2, p. 228f; translated by Michael C. Heller and Marcel Duclos) As man thrived in different regions of the globe, he increased in number, established himself in society with fellow creatures, and finally progressed and became civilized. His delights and his needs increased and became more and more diversified. He developed increasingly varied ways of relating to the society in which he lived; which, among other things, generated increasingly complex personal interests. His inclinations subdivided endlessly, generated new needs that activated themselves beyond the scope of his awareness. These grew into a huge mass of connections that control, outside of his perception, nearly every part of him (Lamarck, 1815, Natural History, p. 278; translated by Michael C. Heller and Marcel Duclos)