ZOOLOGICAL CLASSIFICATION. 
3 
to itself, and each in turn producing similar offspring. Although the 
Cffspring 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 
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 or oak, 
