1058 Influence of circumstances on the Actions of Animals. 
a time, they acquire the singular elongation of the neck, as is seen 
in river birds. 
If some swimming birds, as the swan and the goose, and of which 
the feet are short, have, nevertheless, a very long neck, it is 
because in walking in the water they have the habit of plunging 
their heads below as deeply as they are able, to take the aquatic 
larve and different animalcules which nourish them, and that they 
have no reason for stretching their feet. If an animal, for the 
satisfaction of its wants, should make repeated efforts to elongate 
its tongue, it would acquire considerable length (e.g., the ant-eater, 
the “pic-verd”). If it wants to seize something with the same 
member, then its tongue will divide and become forked. That 
of humming-birds, who seize with-their tongue, and of lizards 
and snakes, who use theirs to feel and investigate bodies which 
are before them, are the proofs of that which I advance. Wants, 
always occasioned by circumstances, and followed by continued 
efforts to satisfy them, are not limited in their results to modify, 
that is to say, to augument or diminish, the extent or the functions 
of these organs, but they succeed in displacing these same organs — 
where certain of these wants make it a necessity. 
Fishes which swim habitually in large bodies of water, having 
occasion to see laterally, have their eyes placed on the sides of the 
head. Their body, more or less flattened according to the species, 
has its edges perpendicular to the plane of the water, and their eyes 
are placed in such a manner that they have an eye on each flattened 
side. But those fishes whose habits involve the necessity of con- 
stantly approaching rivers, particularly rivers little inclined or with 
gentle descent, have been forced to swim with one side downwards 
in order to be able to approach near the edges of the water In 
this situation, receiving more light from above than below, and 
having particular reason for always being attentive to that which 
they find above the water, this want has forced one of their eyes to 
undergo a kind of displacement, and to take the very singular situa- 
tion which is known in soles, turbots, “ limandes,” ete. (the Pleuro- 
nectes and the “Achires”), The situation of these eyes is not 
symmetrical, because there has resulted an incomplete mutation’ 
_ Now this mutation is entirely finished in the rays, where the trans- 
_ verse flattening of the body is altogether horizontal; so with the 
