Page:Natural History Review (1861).djvu/145



Species and Genera, rather than Genera and Species; for the whole system of classification depends, in the first instance, on a right understanding of what is meant by species.

The Species, in the ordinary traditional acceptation of the word, designates the whole of the individuals supposed to be descended from one original plant, or pair of plants. But this definition is practically useless—for we have no means of ascertaining the hereditary history of individual plants—and is considered theoretically incorrect by those who deny the original creation of a certain number of individuals, or pairs of individuals, forming each a parent stock, from which as many constantly distinct races have descended. It has, therefore, been proposed entirely to reject descent as an element in the definition of species, and to consider as such any set of individuals which present, either in their external form, or in their internal structure, or in their biological phenomena, any common character, or combination of characters, distinguishing them from all others. But in nature there are no two individuals exactly alike in every respect. In all collections of individuals, even when the immediate offspring of one parent, peculiarities will be found common to some, and not to all. The species or collection of individuals thus defined, becomes, therefore, as arbitrary as the genus or collection of species, and reduces the rules of classification in the one case, as in the other, to little more than rules of convenience.

Believing, however, as I do, that there exist in nature a certain number of groups of individuals, the limits to whose powers of variation are, under present circumstances, fixed and permanent, I have been in the habit of practically defining the species as the whole of the individual plants which resemble each other sufficiently to make us conclude that they are all, or all, descended from a common parent. Their variations would be such only as we observe among individuals, which we