EVOLUTION. 417 







readily seen to be independent of any variation in con- 

 ditions and can only be explained by differences in the 

 germ-cells from which they arise. Such modifications are 

 called congenital variations. The second, or so-called 

 acquired variations, depend upon external conditions. 

 Two plants from the same lot of seeds will present con- 

 siderable differences provided one be well fed while the 

 other is placed in soil deficient in nutrition. So, too, use 

 and disuse of parts, differences of temperature, of moist- 

 ure, etc., will produce variations. 



Another and an important factor is heredity. Certain of 

 these variations will be inherited. The congenital varia- 

 tions certainly are; there is a question as to the trans- 

 missibility of those which are called acquired. Now, 

 with constant variations in different directions, the con- 

 stant elimination of the unfit, and the inheritance of 

 those variations which have been of benefit, the appear- 

 ance of the species will gradually change; and since 

 different kinds of beneficial variation may occur, the result 

 will be to split the originally homogeneous species into 

 races. A continuation of the process will produce such 

 divergences that the resulting forms must be regarded 

 as distinct species, genera, and higher groups. 



The process is aided by other factors. If the varying 

 forms be in such position that they can interbreed, there 

 will be a constant tendency towards the disappearance of 

 the variation. But if they be isolated, the chances of a 

 favorable variation being perpetuated will be greatly 

 increased. Such isolation is brought about when birds, 

 blown out to sea, have colonized a distant island, or where, 

 as has frequently happened, the sea divides an area of 

 land inhabited by a species into two distinct tracts. 



As a rule this evolution is progressive the new forms 



