Chap. VII.] THEORY OF NATURAL SELECTION. 315 



causes of destruction and by subsequent inter-crossing; 

 and so it is known to be under domestication, unless ab- 

 rupt variations of this kind are specially preserved and 

 separated by the care of man. Hence in order that a 

 new species should suddenly appear in the manner sup- 

 posed by Mr. Mivart, it is almost necessary to believe, 

 in opposition to all analogy, that several wonderfully 

 changed individuals appeared simultaneously within the 

 same district. This difficulty, as in the case of un- 

 conscious selection by man, is avoided on the theory of 

 gradual evolution, through the preservation of a large 

 number of individuals, which varied more or less in any 

 favourable direction, and of the destruction of a large 

 number which varied in an opposite manner. 



That many species have been evolved in an extremely 

 gradual manner, there can hardly be a doubt. The spe- 

 cies and even the genera of many large natural families 

 are so closely allied together, that it is difficult to dis- 

 tinguish not a few of them. On every continent in pro- 

 ceeding from north to south, from lowland to upland, 

 &e., we meet with a host of closely related or representa- 

 tive species; as we likewise do on certain distinct conti- 

 nents, which we have reason to believe were formerly 

 connected. But in making these and the following 

 remarks, I am compelled to allude to subjects hereafter 

 to be discussed. Look at the many outlying islands 

 round a continent, and see how many of their inhabi- 

 tants can be raised only to the rank of doubtful species. 

 So it is if we look to past times, and compare the species 

 which have just passed away with those still living with- 

 in the same areas; or if we compare the fossil species 

 embedded in the sub-stages of the same geological forma- 

 tion. It is indeed manifest that multitudes of species 



