Influence of circumstances on the Actions of Animals, 1055 
bodies; any animal which possesses a plan of organization in which 
hearing is essential, has always occasion to exercise this organ in 
whatever place it inhabits. Therefore, among vertebrate animals, 
one sees none which are deprived of the organ of hearing; but below 
them, when the same organ is wanting, we do not find this sense 
in any of the animals of succeeding classes. It is not thus with the 
organ of sight, for one sees that organ disappear, reappear, and 
disappear again, by reason of the possibility or the impossibility of 
the animals exercising it. In the Mollusques acephales, the great 
development of the mantle has rendered their eyes and their head 
altogether useless, These organs, although taking part in a wider 
plan of organization which comprehends them, have neces- 
sarily disappeared and become obliterated by constant disuse. 
Finally it enters into the plan of organization of reptiles, as of 
other vertebrate animals, that they should have four feet belonging 
to their skeletons. Serpents should have, consequently, also four feet, 
the more so as they do not constitute the last order of reptiles, and 
since they are less related to fishes than are batrachians (frogs, sala- 
manders, etc.) Now, snakes having adopted the habit of crawling 
on the ground, and of hiding themselves under bushes, their bodies, 
in consequence of long-repeated efforts to elongate themselves, in 
order to pass into narrow places, have acquired a considerable 
length, and in no wise proportionate to their thickness. Now, feet 
would have been very useless to these animals, and without employ- 
ment. Long feet would have been a hindrance to creeping, and 
very short feet, even to the number of four, would have been inca- 
pable of moving their bodies. Thus, the disuse of these parts 
having become constant in the races of these animals, has caused 
these same parts to disappear entirely, although they were really in 
the plan of organization of animals of their class. Many insects, 
Which from the natural character of their order, and also of their 
genus, should have wings, lack them more or less completely, from 
disuse. Numbers of Coleoptera, Orthoptera, Hymenoptera, and 
Hemiptera, etc., present examples, their habits never permitting 
them to make use of their wings. But it is not enough to give the 
explanation of the cause which has brought about the condition of 
organs of different animals, conditions which one sees always the — 
Same in those of like species. It is necessary besides to show these 
