INDIVIDUAL DIFFERENTIATION AND ADAPTATION. IOI 



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 like- 

 ness and unlikeness among the great number of individual 

 animals existing on the earth. This may be done by means of 

 an appropriate classification. The differences of structure and 

 function may be superficial or fundamental, but it must be re- 

 membered that all these differences are in some way the out- 

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

 are signs of kinship, or of similar history, or both. The group- 

 ing or classifying of organisms has two objects: (i) con- 

 venience, 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 animals 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 ex- 

 presses 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. 



138. 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 elimina- 

 tion of those whose variations are not suited to the various 

 environments into which the offspring migrate, it will be 

 readily understood that even the descendants of a single pair 

 of organisms will come in time to be noticeably different in 

 form, size, color, and the like. The individuals of a given 

 region will usually be more like each other than like their 

 cousins who have been subjected to some other kind of en- 



