108 ZOOLOGY 



which tend to bring them into harmony with their different 

 environments, it is inevitable that the animals themselves come 

 to be very diverse, no matter how similar they were at the 

 outset. In the discussion of them it therefore becomes neces- 

 sary to devise some means of expressing the degree of likeness 

 and unlikeness among the great number of individual animals 

 existing on the earth. This may be done by means of an ap- 

 propriate classification. The differences of structure and func- 

 tion may be superficial or fundamental, but it must be remem- 

 bered that all these differences are in some way the outcome 

 of the history of the organisms, and that the likenesses are 

 signs of kinship, or of similar history, or both. The grouping 

 or classifying of organisms has two objects: (i) convenience, 

 that is, to make future work easy ; and (2) to express the results of 

 past study. Insomuch as the first motive may predominate the 

 classification may be artificial, that is, may bring together ani- 

 mals that are really not closely related, though possessing a 

 superficial resemblance. The grouping together of bats and 

 birds on the ground of their power of flying, or whales with fishes 

 because of their habitat, would illustrate such a classification. 

 In proportion as classification takes in all the facts known with 

 regard to animals and expresses the relationship of forms classed 

 together, it is said to be natural. Every classification is in some 

 measure artificial since we do not know all the facts concerning 

 the structure or history of any organism. 



142. Terms Used in Classification. From what has been 

 said concerning the power of multiplication in animals, the re- 

 sulting struggle for existence, the variability, and the elimination 

 of those whose variations are not suited to the various environ- 

 ments into which the offspring migrate, it will be readily under- 

 stood that even the descendants of a single pair of organ- 

 isms will come in time to be noticeably different in form, size, 

 color, and the like. The individuals of a given region will usu- 

 ally be more like each other than like their cousins who have 

 been subjected to some other kind of environment. There is 

 thus a need of terms to express the degree of difference which, 

 through these influences, finally characterizes the descendants 



