CALLY CONSIDERED.
The meaning which the scientific Na turalist attaches to the term Species, is not always defined in the same manner, although the notions which the various definitions are intended to convey, are for the most part essentially similar. Thus a species has been described as " a race of animals or of plants, marked by any peculiar character which has always been constant and undeviating ;" it being obvious, from this definition, which carries us backwards from the present to the past, that the first parents or " protoplasts " of such a race must have been distinguished by the same characters as those by which their descendants are now recognised. But, again, this community of parentage is made by Cuvier to constitute the leading idea con veyed by the term ; for he defines a species to be "the collection of all the beings de scended the one from the other, or from com mon parents ; and of those which bear as close a resemblance to these, as they bear to each other." "We are under the necessity," he elsewhere remarks, " of admitting the exist ence of certain forms, which have perpetuated themselves from the beginning of the world, without exceeding the limits first prescribed ; all the individuals belonging to one of these forms, constitute what is termed a species." And M. De Candolle, in like manner, ob serves that " we unite, under the designation of a species, all those individuals which mu tually bear to each other so close a resem blance, as to allow of our supposing that they may have proceeded originally from a single being or a single pair." Thus it appears that, in one mode or another, the fundamental idea of the term, among all those naturalists who admit the "permanence of species," con nects itself with the notion of community of descent. This notion, as M. De Candolle admits, is hypothetical, so far at least as its particular applications are concerned ; since in no one case have we the power either of looking back to the epoch of the first pro duction of a species, or of tracing downwards the whole line of descent from any original pair. Still, as it is the only definition which
conveys the essence of what naturalists or dinarily mean by species, we shall accept it as the basis of our further inquiries ; and shall now point out the mode, in which it is brought into application in the actual study of natural history.
We will suppose the Zoologist to have two new specimens of shells or insects placed before him, or the Botanist to be examining two new specimens of plants. If the con formity between the two is so extremely close, that the differences do not exceed the limits of variation which are commonly seen to prevail in the offspring of a common parentage, he places them in the same species ; because he considers that each may produce a form resembling the other, or may have been pro duced by it, so that there is no sufficient ground for assigning to the two a distinct ancestry. But supposing that the differences should be more strongly marked, and the naturalist should be tempted to assign dif ferent specific names to his two shells, or insects, or plants : in what way is he to diag nose their similarity or diversity of origin ? He forms his judgment, in the first place, by the nature of the characteristic difference ; for this may be of such a kind, that its variability could not be reasonably suspected. Yet this is not a point on which much stress can be laid, when it stands alone ; for although in many groups there are certain characters which present such constancy, that a pre sumption of specific diversity may be fairly entertained if these should exhibit well-marked differences, yet there are too many exceptions to allow such differences to be unhesitatingly admitted as valid specific characters. They may arise, in fact, from three sets of causes ; namely, differences in age and degree of de velopment, differences in the conditions under which the individuals have existed, and tend ency to spontaneous variation inherent in the race. It is necessary, therefore, to ex clude each of these possible sources of error, before the specific diversity of our two objects can be established.