345 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. 
“TI 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 Bu/lea, 
Bulla, and Chiton.” 
In the Supplément a la Distribution générale des 
Animaux (Introduction, p. 342), concerning the real 
