CAUSES OF VARIATION. 2/ 



lives naturally in water of a slight degree of saltness. 

 By gradually raising the percentage of salt this ex- 

 perimenter succeeded in transforming this species 

 into an entirely different one (A. Salind), and by 

 reversing the process he transformed the latter 

 species into the former. Nor was this all. Still 

 further freshening the water by gradual changes, he 

 succeeded in transforming this same species into still 

 another, which was so very different from the first 

 that it had previously been ranked as a distinct 

 genus (Branchippus). These changes took place 

 slowly, several generations being required for the 

 complete transformation, showing that the indi- 

 vidual variations were transmitted from one genera- 

 tion to another, increasing in importance with each 

 generation. All of these species had been known 

 before, and had always been considered as distinct 

 species and genera. Here was actual proof of their 

 genetic connection with each other. It is important 

 to notice that this naturalist was led to carry on his 

 experiments by seeing them performed by nature on 

 a grand scale by the gradual freshening of a salt- 

 water lake. This instance is, therefore, enough to 

 prove that not only do variations occur under 

 nature, but that they may by being transmitted be- 

 come sufficiently great to give rise to varieties which 

 all naturalists rank as species. 



Causes of Variation. 



For every one of these variations, whether it be 

 tfiinute or great in amount, there must be an ade- 

 quate cause. But in regard to the nature of these 



