1302 



VARIETIES OF MANKIND. 



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. 



1. It is now universally admitted that the 

 cases are extremely numerous, in which diver- 

 sities of age have led to the establishment of 

 species which have no existence in nature ; 

 the forms thus distinguished being those of 

 the same species in different grades of de- 

 velopment. The more our knowledge of the 

 history of the lower tribes of animals in- 

 creases, the m ore is it found that metamor- 



phosis is with them the rule, and not the ex- 

 ception ; so that the cases seem comparatively 

 rare, in which an invertebrated animal at its 

 emersion from the egg possesses the cha- 

 racters that serve to distinguish it in its adult 

 condition. And just as the larva, pupa, and 

 imago states of any insect, are all compre- 

 hended in a complete account of the species, so 

 must we rank the extraordinary diversities of 

 form presented by the Medusce or the Balani, 

 in the early period of their lives, as coming 

 within the limits of their specific definitions. 

 It is obvious that this source of fallacy can 

 only be completely avoided, when we have 

 obtained an acquaintance with the whole his- 

 tory of the life of any individual, from its com- 

 mencement to its dissolution, and are thus 

 enabled to say positively what are, and what 

 are not, alterations producible by age. Where 

 this knowledge cannot be acquired, the only 

 safe basis on which the naturalist can pro- 

 ceed, is that which is derived from a know- 

 ledge of these phenomena as presented in 

 the most nearly allied forms ; and yet this 

 often fails, as in the case of the Astacus fluvia- 

 tilis (river cray-fish), and Gecarcinus (land 

 crab), which undergo no change that can be 

 called a metamorphosis, notwithstanding that 

 in all other Macrourous and Brachyourous 

 Decapods yet observed, a real metamorphosis 

 takes place. Even in the case of extinct 

 species, the history of whose life can never 

 become known to us by any other means 

 than by the preservation of their remains in 

 different stages of growth, the careful com- 

 parison of a sufficient series of these remains 

 will sometimes establish a strong probability, 

 if not a positive certainty, as to their mutual 

 relationship : thus, M. Barrande, of Prague, 

 has succeeded in showing that it is next to 

 certain that no fewer than eighteen forms of 

 Trllobites, which have been described as dis- 

 tinct species and ranked under ten different 

 genera, are really the successive forms of one 

 and the same species ; the differences which 

 they present both in size and conformation 

 being analogous to those that we see in the 

 existing tribes most nearly allied to them, 

 and the whole series constituting one con- 

 tinuous succession. Instances in which new 

 species have been erected among the higher 

 classes of animals, especially among Birds, for 

 the reception of individuals whose differences 

 were only seasonal, have been so frequently 

 recorded, that it is sufficient here to mention 

 them. It is obvious that such errors can 

 only be corrected by a knowledge of the 

 seasonal changes which the species is liable 

 to undergo. Of this source of difficulty in 

 the discrimination of species, we need take no 

 account in our future inquiries ; for although, 

 in the Orang and Chimpanzee, the alteration 

 in the conformation of the cranium which 

 takes place at the period of second dentition, 

 is so very decided, that it formerly gave rise 

 to much confusion, which has only disap- 

 peared before a fuller knowledge of the 

 history of these animals, yet no change of 

 such magnitude occurs in Man ; and of the 



