26 EVOLUTION OF TO-DA Y. 



only produce changes in color. Hardly a species of 

 animals exists which does not have two or more 

 well-marked varieties, sometimes referable to differ- 

 ences in conditions, but more frequently not, since 

 they may exist side by side. Now since these varie- 

 ties are all descended from a common stock, their 

 existence is proof that a great amount of change 

 is constantly taking place in the structure of our 

 species. 



A word as to the amount of the individual varia- 

 tions. The changes which occur in single individuals 

 are of all grades, great and small. Sometimes they 

 are so minute that only a practised eye can see 

 them, while on the other hand they may be so great 

 as to be almost incredible. The pug dog, the short- 

 faced tumbler pigeons, the hook-billed ducks ap- 

 peared suddenly in nearly the same state in which 

 we now find them. We thus see that the modifica- 

 tion of species through variation may be a slow 

 continual process of accumulation of minute varia- 

 tions ; or it may be an interrupted process occurring 

 by a series of greater or lesser jumps. It is true that 

 it is seldom possible in the case of wild animals to 

 determine by direct observation that these individual 

 variations are transmitted from generation to genera- 

 tion. But the existence of varieties in nature which 

 do breed true is a sufficient proof that such is the 

 case. Moreover in some instances it has been dK 

 rectly observed. Perhaps the best instance of the 

 kind is a series of experiments of a Russian natural- 

 ist, Schmankewitsch. These experiments were upon 

 a species of Crustacea (Artemia Milhausenii) which 



