30 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 compared 

 with the eye of a fly, is found to be not only more perfect, 

 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 acquainted 

 with the instruments which execute them. The study of 

 the functions or uses of organs, therefore, requires an exam- 

 ination of their structure ; they must never be disjoined, 

 and must 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 even 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 botli of them serve for flight. 

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

 hereafter 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 different purposes, the one for flight, and the other 

 for climbing, they are both constructed on the same plan. 

 Accordingly, the bird is more nearly allied to the monkey 

 than to the butterfly, though they both have in common 

 the faculty of flight. Affinities, and not analogies, therefore, 

 must guide us in the arrangement of animals. 



