THEORY OF NATURAL SELIXTION 259 



changed as abruptly as have occasionally domestic races, and 

 for entirely disbelieving that they have changed in the won- 

 derful manner indicated by Mr. Mivart, are as follows. Ac- 

 cording to our experience, abrupt and strongly marked vari- 

 ations occur in our domesticated productions, singly and at 

 rather long intervals of time. If such occurred under na- 

 ture, they would be liable, as formerly explained, to be lost 

 by accidental causes of destruction and by subsequent inter- 

 crossing; and so it is known to be under domestication, un- 

 less abrupt 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 supposed 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 dif- 

 ficulty, as in the case of unconscious selection by man, is 

 avoided on the theory of gradual evolution, through the pres- 

 ervation 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 species 

 and even the genera of many large natural families are so 

 closely allied together, that it is difficult to distinguish not a 

 few of them. On every continent in proceeding from north 

 to south, from lowland to upland, &c., we meet with a host 

 of closely related or representative species ; as we likewise 

 do on certain distinct continents, 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 inhabitants 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 within the same areas; or 

 if we compare the fossil species embedded in the sub-stages 

 of the same geological formation. It is indeed manifest that 

 multitudes of species are related in the closest manner to 

 other species that still exist, or have lately existed ; and it will 

 hardly be maintained that such species have been developed 



