244 Darwin, and after Darwin. 



germ-cells and their contained germ-plasm, because as 

 follows from sundry facts the molecular structure of the 

 germ-plasm must be very difficult to change, yet it is by no 

 means to be implied that this structure may not possibly be 

 altered by influences of the same kind continuing for a very 

 long time. Thus it seems to me the possibility is not to be 

 rejected, that influences continued for a long time, that is, 

 for generations, such as temperature, kind of nourishment, 

 c., which may affect the germ-cells as well as any other 

 part of the organism, may produce a change in the constitu- 

 tion of the germ-plasm. But such influences would not then 

 produce individual variation, but would necessarily modify in 

 the same way all the individuals of a species living in a certain 

 district. It is possible, though it cannot be proved, that 

 many climatic varieties have arisen in this manner." 



So far, then, we have testimony to this point, as it 

 were, from a reluctant witness. But if we have no 

 theory involving the "stability of germ-plasm" to 

 maintain, we can scarcely fail to see how susceptible 

 the germ-plasm is likely to prove to changed con- 

 ditions of life. For we know how eminently sus- 

 ceptible it is in this respect when gauged by the 

 practical test of fertility ; and as this is but an expres- 

 sion of its extraordinarily complex character, it would 

 indeed be surprising if it were to enjoy any immunity 

 against modification by changed conditions of life. 

 We have seen in the foregoing chapter how fre- 

 quently and how considerably somatogenetic changes 

 are thus caused, so as to produce " somatogenetic 

 species" or, where we happen to know that the 

 changes are not hereditary, " climatic varieties." But 

 the constitution of germ-plasm is much more complex 

 than that of any of the structures which are developed 

 therefrom. Consequently, the only wonder is that 

 hitherto experimentalists have not been more successful 



