686 Studies in the Theory of Descent. 



ever, imagine on the contrary, that a large group 

 of individuals is affected by the same influences 

 in fact by such influences as the remaining indi- 

 viduals of the species are not exposed to : this 

 group of individuals would then vary in a nearly 

 similar manner, since both factors of variation, viz. 

 the external influence and the physical consti- 

 tution, are equal or nearly so. Such local 

 variations would first become prominent when 

 the same external influence had acted upon a 

 scries of generations, and the minima of variation 

 produced in the individual by the once-exerted 

 action of the cause inciting change had become 

 augmented by heredity. Transformations of 

 some importance (up to the form-value of spe- 

 cies) can thus arise simply by the direct action 

 of the environment, in the same way as that in 

 which individual differences are produced only 

 the latter fluctuate from generation to generation, 

 since the inciting influences continually change ; 

 whilst, in the former, the constant external cause 

 inciting modification always reproduces the same 

 variation, so that an accumulation of the latter 

 can take place. Climatic varieties can be thus 

 explained. 



A more efficacious augmentation of the varia- 

 tions arising in the single individual is certainly 

 brought about by the indirect action of the envi- 

 ronment upon the organism. It is not here my 

 intention to explain once more the processes of 



