232 THE AMEPdCAN NATURALIST [Vol. XLII 



ample, it may be supposed that a rose, or a few species 

 of rose, were originally created. In the course of time 

 these have produced endless varieties, some of which, 

 depending for a long series of ages upon permanent 

 peculiarities of soil or climate, have been in a manner 

 fixed, acquiring a constitution and physiognomy of their 

 own. Such supposed varieties have again intermixed 

 with each other, producing other forms, and so the opera- 

 tion has proceeded. But as it is impossible, at the 

 present day, to determine which was the original or orig- 

 inals, from which all the roses of our own time have 

 proceeded, or even whether they were produced in the 

 manner I have assumed; and as the forms into which 

 they divide are so peculiar as to render a classification 

 of them indispensable to accuracy of language; it has 

 become necessary to give names to certain of those forms, 

 which are called species. Thus it seems that there are 

 two sorts of species: the one, called natural species, de- 

 termined by the definition given above; and the other, 

 called botanical species, depending only upon the ex- 

 ternal character of the plant. The former have been 

 ascertained to a very limited extent; of the latter nearly 

 the whole of systematic botany consists. In this sense 

 a species may be defined to be ''an assemblage of indi- 

 viduals agreeing in all the essential characters of vegeta- 

 tion and fructification." Here the whole question lies 

 with the word essential. "VAH^iat is an essential character 

 of a species? This will generally depend upon a prone- 

 ness to vary, or to be constant in particular characters, 

 so that one class of characters may be essential in one 

 genus, another class in another genus ; and these points 

 can be only determined by experience. Thus, in the 

 genus Dahlia, the form of the leaves is found to be sub- 

 ject to great variation; the same species producing from 

 seed, individuals, the form of whose leaves vary in a 

 very striking manner; the form of the leaves is, there- 

 fore, in Dahlia, not a specific character. In like manner, 

 in Rosa, the number of prickles, the surface of the fruit. 



