"I conceive, for example, that a _gasteropod mollusc_, which, as it crawls along, finds the need of feeling the bodies in front of it, makes efforts to touch those bodies with some of the foremost parts of its head, and sends to these every time supplies of nervous fluids, as well as other fluids--I conceive, I say, that it must result from this reiterated afflux towards the points in question that the nerves which abut at these points will, by slow degrees, be extended. Now, as in the same circumstances other fluids of the animal flow also to the same places, and especially nourishing fluids, it must follow that two or more tentacles will appear and develop insensibly under those circumstances on the points referred to. "This is doubtless what has happened to all the races of _Gasteropods_, whose wants have compelled them to adopt the habit of feeling bodies with some part of their head. "But if there occur, among the _Gasteropods_, any races which, by the circumstances which concern their mode of existence or life, do not experience such wants, then their head remains without tentacles; it has even no projection, no traces of tentacles, and this is what has happened in the case of _Bullæa_, _Bulla_, and _Chiton_." In the _Supplément à la Distribution générale des Animaux_ (Introduction, p. 342), concerning the real order of origin of the invertebrate classes, Lamarck proposes a new genealogical tree. He states that the order of the animal series "is far from simple, that it is branching, and seems even to be composed of several distinct series;" though farther on (p. 456) he adds: "Je regarde _l'ordre de la production_ des animaux comme formé de deux séries distinctes. "Ainsi, je soumets à la méditation des zoologistes l'ordre présumé de la _formation_ des animaux, tel que l'exprime le tableau suivant:" In the matter of the origin of instinct, as in evolution in general, Lamarck appears to have laid the foundation on which Darwin's views, though he throws aside Lamarck's factors, must rest. The "inherited habit" theory is thus stated by Lamarck. Instinct, he claims, is not common to all animals, since the lowest forms, like plants, are entirely passive under the influences of the surrounding medium; they have no wants, are automata. "But animals with a nervous system have _wants_, _i.e._, they feel hunger, sexual desires, they desire to avoid pain or to seek pleasure, etc. To satisfy these wants they contract habits, which are gradually transformed into so many propensities which they can neither resist nor change. Hence arise habitual actions and special _propensities_, to which we give the name of _instinct_.