350 ANIMAL BIOLOGY 



are essentially similar. It is such a group of similar individuals 

 that is regarded by the biologist as a species. But it is difficult to 

 formulate a satisfactory brief definition of a species, unless perhaps 

 it be "a group of individuals that do not differ from one another in 

 excess of the limits of 'individual diversity,' actual or assumed." 

 So with but slight exaggeration it may be said that a species is 

 largely a concept of the human mind: a somewhat arbitrary con- 

 venience. The real unit in nature is the individual animal or plant, 

 and an understanding of the differences between individuals should 

 give us the key to the differences between species. In the final 

 analysis, the problem of the origin of species is a problem in 

 genetics. (Figs. 236, 280.) 



This seemingly obvious point of view has but relatively recently 

 been clearly grasped by biologists, and the species rather than 

 the individual has loomed large in the discussions of how plants 

 and animals -came to be what they are to-day. As a matter of 

 fact, during the eighteenth century the greatest student of plant 

 and animal classification, Linnaeus, emphasized the idea that 

 each species represents a distinct thought of the Creator, and 

 that the object of classification is to arrange species in the order of 

 the Creator's consecutive thoughts. This viewpoint is somewhat 

 whimsically expressed by the old naturalist who, finding a beetle 

 which did not seem to agree exactly with any species in his collec- 

 tion, solved the difficulty by crushing the unorthodox individual 

 under his foot. His credulity surely would have been strained by 

 the estimate of modern entomologists that if all the species of In- 

 sects were known they would total upward of three million. 



We may consider, then, that the consensus of opinion up to the 

 middle of the last century was overwhelmingly on the side of 

 special creation and fixity of species, and therefore against 

 the idea occasionally advanced by men, as it now appears, ahead 

 of their times, that descent with change is the true explana- 

 tion of the origin of the diverse forms of plants and animals. But, 

 as nearly everyone knows, a complete reversal of opinion has oc- 

 curred since 1860 — to-day professional scientists and most edu- 

 cated laymen accept organic evolution. And we have accepted 

 it in the preceding sections of this work; but if this appears to 

 have been prejudging the question, the explanation is that the 

 genetic connection of organisms is the guiding principle of all 

 modern biology. The mere fact that an unbiased presentation of 



