34 
MEMOIR OP LAMARCK. 
to the point in question, there would follow a gra- 
dual expansion of the nerves which terminate there ; 
and as the nutritious and other juices likewise flow 
to the same point, it must necessarily happen that 
two or four tentacula would insensibly be produced. 
This is no doubt what happens in regard .to all the 
gasteropodc tribes, whose wants occasion the habit 
of feeling bodies by touching them with the parts of 
their head ; and when such wants arc not felt, the 
head remains destitute of tentacula, as may be seen 
in other instances, &c. * In like manner it is the 
desire and the attempt to swim, that had, in time, 
the effect of extending the skin that unites the toes 
of many aquatic birds, and thus the web-foot of the 
gull and duck were at last produced. The necessity 
of wading in search of food, accompanied with the 
desire to keep their bodies from coming in contact 
with the water, has lengthened to these present 
dimensions, the legs of the grail® or wading-birds ; 
while the desire of flying has converted the arms of 
all birds into wings, and their hairs and scales into 
feathers. Changes of this nature may appear to us 
contrary to what falls under our observation, which 
leads us to suppose that the specific forms of animals 
are constant ; but this error is entirely owing to the 
difficulty we experience in embracing a considerable 
portion of time within the scope of our observations. 
It is from this cause that we cannot he ourselves 
witnesses of these changes, and neither liistory nor 
written observations extend to sufficiently remote a 
* Animaux sans rertZlres, vol. i. p. 188, 189. 
