ZOOLOGICAL CLASSIFICATION. 3 



to itself, and each in turn producing similar offspring. Although the 

 offspring is never exactly like either parent, the degree of variation in 

 a single generation is slight. And thus we find that there exist large 

 numbers of individuals which very closely resemble each other. Such 

 a collection of individuals is termed in popular language a kind, in 

 scientific language a species. Thus the kind of pine trees known as 

 pitch-pine is a species; and scrub-pine, still another. In the same 

 way the name sparrow-hawk indicates a kind or species of hawk; and 

 pigeon-hawk, another species. 



Roughly speaking, a species is a collection of individuals which 

 resemble each other as closely as the offspring of a single 

 parent. For example, if any two pitch-pines be studied, nothing will 

 be found to indicate that they may not have sprung from seeds 

 grown upon the same tree. On the other hand, if a pitch-pine and 

 a white-pine be carefully compared, they will be found so different 

 that no competent observer would believe that they had a common 

 parent. 



Unfortunately this mode of defining the limits of a species cannot 

 be depended upon. Many instances are known where forms of 

 animals or plants living in widely-separated regions differ so greatly 

 that they have been considered distinct species until more extended 

 collections in the intermediate regions have brought to light series 

 of intermediate forms, which connect the two so-called species so 

 closely that it is impossible to say where the one ends and the other 

 begins. 



The only definite way of determining whether two forms are 

 specifically distinct is to determine whether they naturally interbreed 

 or not. We find among wild animals a sort of race prejudice which 

 keeps the members of different species from pairing, although they 

 may do so when demoralized by domestication. Except in the case 

 of very-closely-allied species, the pairing of individuals of different 

 species results in no offspring or in the production of sterile offspring. 



This grouping of individuals into species not only facilitates our 

 study of Natural History, but expresses certain important facts of 

 inheritance and reproduction. A second and somewhat similar step 

 is made by grouping species into genera. 



We find that there exist groups of closely-allied species, species 

 that resemble each other in all of the more important characters, and 

 differ among themselves only in what are known as the specific 

 characters. Such a group of species is termed a genus. Thus all the 

 different species of pine taken together constitute the genus pine, or 

 Pinus, as it is termed by botanists. There are many species of oak, 



