PRINCIPLES OF ZOLOOGT. 



at a great distance, and when compared with the eye of a fly, 

 is found to be not only more complicated, but constructed 

 on an entirely different plan. It is the same with every other 



organ. 



15. We understand the faculties of animals, and appro 

 ciate their value, just in proportion as we become acquainted 

 with the instruments which execute them. The study of the 

 functions or uses of organs therefore requires an examination 

 of their structure ; Anatomy and Physiology must never be 

 disjoined, and ought to precede the systematic distribution of 

 animals into classes, families, genera, and species. 



16. In this general view of organization, we must ever 

 bear in mind the necessity of carefully distinguishing be- 

 tween affinities and analogies, a fundamental principle re- 

 cognized even by Aristotle, the founder of scientific Zoology. 

 Affinity or homology is the relation between organs or parts 

 of the body which are constructed on the same plan, how- 

 ever much they vary in form, or serve for different uses. Ana- 

 logy, on the contrary, indicates the similarity of purposes or 

 functions performed by organs of different structure. 



1 7. Thus, there is an analogy between the wing of a bird 

 and that of a butterfly, since both of them serve for flight. 

 But there is no affinity between them, since, as we shall here- 

 after see, they differ totally in their anatomical relations. On 

 the other hand, there is an affinity between the bird's wing 

 and the hand of a monkey, since, although they serve for dif- 

 ferent purposes, the one for climbing, and the other for flight, 

 yet they are constructed on the same plan. Accordingly, the 

 bird is more nearly allied to the monkey than to the butterfly, 

 though it has the faculty of flight in common with the latter. 

 Affinities, and not analogies, therefore, must guide us in the 

 arrangement of animals. 



18. Our investigations should not be limited to adult 

 animals, but should also be directed to the changes which 

 they undergo during the whole course of their development. 

 Otherwise, we shall be liable to exaggerate the importance of 

 certain peculiarities of structure which have a predominant 

 character in the full-grown animal, but which are shaded off, 

 and vanish, as we revert to the earlier periods of life. 



19. Thus, for example, by regarding only adult indivi- 

 duals, we might be induced to divide all animals into two 



