ORGANIZATION OF ANIMALS. 183 



another pair, unfitted for such a function, here become the 

 special organs of prehension ; a third series of members or 

 limbs is devoted to locomotion only, and of these, some are 

 used only in swimming, others in walking on the firm 

 ground. 



This tendency on the part of nature to appropriate the 

 same part of the animal economy to different functions, 

 according to the wants of the animal, rather than to create 

 for each species parts entirely new, reveals itself also when 

 we compare with each other, species destined to live differently. 

 Already in the vertebrata we have seen how, out of the same 

 elements, nature constructs a limb or arm, an instrument of 

 prehension or one of mere locomotion and support, a fin or 

 wing ( 200, &c.). Such adaptations are no less curious in 

 insects, to which we shall return ; we limit ourselves here to 

 the remark, that anatomists give the name of analogues, or 

 analogous parts, to the organs which, however varied may be 

 their uses in the economy, are yet obviously composed of the 

 same anatomical elements. 



351. It is in general by means of such transformations 

 that nature varies most the structure of animals. She seems 

 to have been desirous of producing the greatest variety pos- 

 sible with the smallest elementary means essentially dif- 

 ferent ; and to have had recourse to the creation of parts 

 entirely new, only after having exhausted the combinations to 

 which parts already existing in other organisms could lend 

 themselves. This disposition is connected with another ten- 

 dency, namely, the tendency to uniformity of the organic 

 composition. It would be absurd to assert that all beings 

 are formed upon one plan, and constructed out of the same 

 materials; but if we examine the structure of one of the 

 more complex organisms, we shall find that the lower are 

 characterized by a modification of the larger features of the 

 former, by an omission of some parts, or by the existence of 

 organs of which the former have been deprived. A frog, for 

 example, differs greatly from man, and yet in its general 

 outline may easily be traced the indications of the plan 

 upon which man has been constructed. When the entire 

 animal kingdom is contemplated, it becomes impossible to 

 perceive this unity of plan and of organization ; but, by restrict- 

 ing the field of view, it becomes evident that, notwithstanding 

 the immense number of animals, all have been constructed 

 upon a few primitive types. Now, it is by the consideration 



