INFLUENCE OF ENVIRONMENT 121 



incomplete mutation. Now this mutation is entirely completed in 

 the skates, in which the transverse flattening of the body is altogether 

 horizontal, like the head. Accordingly the eyes of skates are both 

 situated on the upper surface and have become symmetrical. 



Snakes, which crawl on the surface of the earth, chiefly need to see 

 objects that are raised or above them. This need must have had its 

 effect on the position of the organ of sight in these animals, and accord- 

 ingly their eyes are situated in the lateral and upper parts of their 

 head, so as easily to perceive what is above them or at their sides ; but 

 they scarcely see at all at a very short distance in front of them. They 

 are, however, compelled to make good the deficiency of sight as regards 

 objects in front of them which might injure them as they move forward. 

 For this purpose they can only use their tongue, which they are 

 obliged to thrust out with all their might. This habit has not only 

 contributed to making their tongue slender and very long and con- 

 tractile, but it has even forced it to undergo division in the greater 

 number of species, so as to feel several objects at the same time ; 

 it has even permitted of the formation of an aperture at the extremity 

 of their snout, to allow the tongue to pass without having to separate 

 the jaws. 



Nothing is more remarkable than the effects of habit in herbivorous 

 mammals. 



A quadruped, whose environment and consequent needs have for 

 long past inculcated the habit of browsing on grass, does nothing but 

 walk about on the ground ; and for the greater part of its life is 

 obliged to stand on its four feet, generally making only few or moderate 

 movements. The large portion of each day that this kind of animal 

 has to pass in filling itself with the only kind of food that it cares for, 

 has the result that it moves but little and only uses its feet for support 

 in walking or running on the ground, and never for holding on, or 

 climbing trees. 



From this habit of continually consuming large quantities of food- 

 "material, which distend the organs receiving it, and from the habit 

 of making only moderate movements, it has come about that the body 

 of these animals has greatly thickened, become heavy and massive 

 and acquired a very great size : as is seen in elephants, rhinoceroses, 

 oxen, buffaloes, horses, etc. 



The habit of standing on their four feet during the greater part of 

 the day, for the purpose of browsing, has brought into existence a 

 thick horn which invests the extremity of their digits ; and since 

 these digits have no exercise and are never moved and serve no other 

 purpose than that of support like the rest of the foot, most of them 

 have become shortened, dwindled and, finally, even disappeared. 



