DARWINISM CHAP. 



produce fertile individuals, and which reproduce themselves 

 by i;enenitiou, in such a manner that we may from analogy 

 suppose them all to have sprung from one single individual." 

 And the zoologist Swainson gives a somewhat similar defini- 

 tion : " A species, in the usual acceptation of the term, is an 

 animal which, in a state of nature, is distinguished by certain 

 peculiarities of form, size, colour, or other circumstances, from 

 another animal. It propagates, 'after its kind,' individuals 

 perfectly resembling the parent; its peculiarities, therefore, 

 are permanent." ^ 



To illustrate these definitions we will take two common 

 English birds, the rook (Corvus frugilegus) and the crow 

 (Corvus corone). These are distinct species^ because, in the first 

 place, they always differ from each other in certain slight 

 peculiarities of structure, form, and habits, and, in the second 

 place, because rooks always produce rooks, and crows produce 

 crows, and they do not interbreed. It was therefore con- 

 cluded that all the rooks in the world had descended from a 

 single pair of rooks, and the crows in like manner from a 

 single pair of crows, while it was considered impossible that 

 crows could have descended from rooks or vice versa. The 

 "origin" of the first pair of each kind was a mystery. 

 Similar remarks may be applied to our two common plants, 

 the sweet violet (Viola odorata) and the dog violet (Viola 

 canina). These also produce their like and never produce 

 each other or intermingle, and they were therefore each 

 supposed to have sprung from a single individual whose 

 " origin " was unknown. But besides the crow and the rook 

 there are about thirty other kinds of bii'ds in various parts of 

 the world, all so much like our species that they receive the 

 common name of crows ; and some of them differ less from 

 each other than does our crow from our i-ook. These are all 

 species of the genus Corvus, and were therefore believed to 

 have been always as distinct as they are now, neither more 

 nor less, and to have each descended from one pair of ances- 

 tral crows of the same identical species, which themselves had 

 an unknown "origin." Of violets there are more than a 

 hundred diff'erent kinds in various parts of the Avorld, all 

 diffeiing very slightly from each other and forming distinct 

 ^ Geography ami Class ijicalion 0/ Animals, p. 350. 



