348 LAMARCK, HIS LIFE AND WORK 



existing, or makes efforts for the formation of those 

 which would not have existed there, and which a con- 

 tinual want would therefore render necessary — I 

 should have had doubts as to the reality of the law 

 which I have just indicated. 



" But, although it may be very difficult to verify 

 this law by observation, I have no doubt as to the 

 grounds on which I base it, the necessity of its exist- 

 ence being involved in that of the third law, which is 

 now well established. 



" 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 tenta- 

 cles ; it has even no projection, no traces of tentacles, 

 and this is what has happened in the case of BuUcea, 

 Bulla, and Chiton." 



In the Supplement h la Distribution g/n^rale des 

 Animaux (Introduction, p. 342), concerning the real 



