460 ORIGIN OF SPECIES 



In confirmation of this view, let us glance at the classifi- 

 cation of varieties, which are known or believed to be 

 descended from a single species. These are grouped under 

 the species, with the sub-varieties under the varieties; and 

 in some cases, as with the domestic pigeon, with several 

 other grades of difference. Nearly the same rules are fol- 

 lowed as in classifying species. Authors have insisted on 

 the necessity of arranging varieties on a natural instead of 

 an artificial system; we are cautioned, for instance, not to 

 class two varieties of the pine-apple together, merely because 

 their fruit, though the most important part, happens to be 

 nearly identical ; no one puts the Swedish and common turnip 

 together, though the esculent and thickened stems are so 

 similar. Whatever part is found to be most constant, is used 

 in classing varieties; thus the great agriculturist Marshall 

 says the horns are very useful for this purpose with cattle, 

 because they are less variable than the shape or colour of the 

 body, &c. ; whereas with sheep the horns are much less serv- 

 iceable, because less constant. In classing varieties, I 

 apprehend that if we had a real pedigree, a genealogical 

 classification would be universally preferred; and it has been 

 attempted in some cases. For we might feel sure, whether 

 there had been more or less modification, that the principle 

 of inheritance would keep the forms together which were 

 allied in the greatest number of points. In tumbler pigeons, 

 though some of the sub-varieties differ in the important 

 character of the length of the beak, yet all are kept together 

 from having the common habit of tumbling; but the short- 

 faced breed has nearly or quite lost this habit; nevertheless, 

 without any thought on the subject, these tumblers are kept 

 in the same group, because allied in blood and alike in some 

 other respects. 



With species in a state of nature, every naturalist has in 

 fact brought descent into his classification; for he includes 

 in his lowest grade, that of species, the two sexes ; and how 

 enormously these sometimes differ in the most important 

 characters, is known to every naturalist : scarcely a single 

 fact can be predicated in common of the adult males and 

 hermaphrodites of certain cirripedes, and yet no one dreams 

 of separating them. As soon as the three Orchidean forms, 



