V. The Affinities of Animals, etc. 53 



V. The AflB.nities of Animals.— Classification.— 

 The Doctrine of Descent. 



The innumerable animals inhabiting the earth have been separated 

 into many principal groups^ and these again into smaller and smaller 

 sections. This graduated arrangement is not arbitrary, but is based 

 upon the degree of conformity of animals to one another, so that the 

 most subordinate orders comprised in a division present the greatest 

 conformity. The theory of species will now be discussed in some 

 ■detail. 



In the first place, all those individuals are ranked as the same 

 species which show a close correspondence in all details of their 

 structure at the same age : further, all those individuals which pair 

 with one another, under normal conditions, and produce fertile 

 ■offspring, whether they resemble each other or not : finally, also, 

 the offspring, which may or may not be like the parents. All the 

 female Hares in England, for instance, belong to one species, for they 

 exhibit at the same age a striking similarity in all parts of the body : 

 to the same species, also, belong all the male English Hares, in spite 

 •of the difference in the genitalia, since they readily copulate with the 

 females, and produce fertile offspring: finally, the young ones, 

 which, in this case, do not differ from the progenitors. In cases of 

 metagenesis and heterogony, on the other hand, the offspring differ 

 ■considerably from the parents ; every second, or third, or fourth 

 generation is alike, but differs from the intermediate ones ; all, 

 however, belong to the same species. 



The conception of species depends, consequently, upon three 

 points : 1 . similarity of structure ; 2. possibility of interbreeding ; 

 •3. genetic connection (descent). The similarity of different specimens 

 of the same species is, however, not absolute, even if those peculiarities 

 which result from differences of sex, of age, or of generations (in 

 ■alternation of generations, and heterogony) are overlooked. Identical 

 individuals are never seen ; a careful examination always brings out 

 differences, excepting in the very smallest creatures, where they 

 ■cannot be seen : species vary, as it is said, in a greater or a less 

 degree. The variation is usually not striking ; externally it is generally 

 restricted to slight differences of colour, shape, to relative size of the 

 ■same part, or to the absolute size or weight of the whole animal. 

 Internally, there are corresponding small differences to be observed. 

 Each large mammalian species presents obvious examples of these 

 ordinary variations. But sometimes, variations become more con- 

 .spicuous : for instance, Foxes are usually white underneath, but 

 .specimens have been found with black undersides, or with black cross- 



