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 interieur_. 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 interieur_, 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 interieur_ 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 interieur_. The _sentiment interieur_ 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 _Echelle des etres_. 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.