LAMARCK'S THEORY OF DESCENT 



309 



we see none deprived of their organs of hearing ; but 

 in the groups below them, when the same organs are 

 once wanting, we do not again find them. 



" It is not so with the organ of sight, for we see 

 this organ disappear, reappear, and again disappear, 

 in proportion to the possibihty or impossibility of 

 the animal's exercising it. 



" In the acephalous molluscs, the great development 

 of the mantle of these molluscs has rendered their 

 eyes and even their head entirely useless. These 

 organs, also forming a part of a plan of organization 

 which should comprise them, have disappeared and 

 atrophied from constant lack of use. 



" Finally, it is a part of the plan of organization of 

 reptiles, as in other vertebrate animals, to have four 

 legs appended to their skeleton. The serpents should 

 consequently have four, though they do not form the 

 lowest order of reptiles, and are not so near the fishes 

 as the batrachians (the frogs, the salamanders, etc.). 



" However, the serpents having taken up the habit 

 of gliding along the ground, and of concealing them- 

 selves in the grass, their body, owing to continu- 

 ally repeated efforts to elongate itself so as to pass 

 through narrow spaces, has acquired a considerable 

 length disproportionate to its size. Moreover, limbs 

 would have been very useless to these animals, and 

 consequently would not have been employed : because 

 long legs would have interfered with their need of 

 gliding, and very short legs, not being more than four 

 in number, would have been incapable of moving 

 their body. Hence the lack of use of these parts 

 having been constant in the races of these animals, 

 has caused the total disappearance of these same 

 parts, although really included in the plan of organi- 

 zation of the animals of their class. 



" Many insects which by the natural character of 

 their order, and even of their genus, should have 

 wings, lack them more or less completely from dis- 



