6 SPHERE AND FUNDAMENTAL 



ulty of perceiving light, without the power of distinguishing 

 objects. The keen eye of the bird, on the contrary, dis- 

 cerns minute objects at a great distance, and when com- 

 pared 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 appre- 

 ciate their value, just in proportion as we become ac- 

 quainted with the instruments which execute them. The 

 study of the functions or uses of organs therefore requires 

 an examination of their structure ; they must never be dis- 

 joined, and must precede the systematic distribution of ani- 

 mals 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. 

 Analogy or liomology is the relation between organs or parts 

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

 ever much they vary in form, but which serve for very dif- 

 ferent uses. Analogy, on the contrary, indicates the simi- 

 larity of purposes or functions performed by organs of dif- 

 ferent structure. 



17. 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. 



