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 

 gasteropode tribes, whose wants occasion the habit 

 of feeling bodies by touching them with the parts of 

 their head ; and when such wants are 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 gTalhe 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 be ourselves 

 witnesses of these changes, and neither history nor 

 written observations extend to sufficiently remote a 

 * Animaux sans verttbres, vol. i. p. 188, 189. 



