^ THEORIES OF HEREDITY ig 



latter only arise when they have been preceded 

 by corresponding changes in the former, then we 

 can, up to a certain point, understand the princi- 

 ple of heredity ; or, at any rate, we can conceive 

 that the human mind may at some time be capa- 

 ble of understanding it. We may at least main- 

 tain that it has been rendered intelligible, for we 

 can thus trace heredity back to growth ; we can 

 thus look upon reproduction as an overgrowth of 

 the individual, and can thus distinguish between 

 a succession of species and a succession of indi- 

 viduals, because in the latter succession the germ- 

 plasm remains similar, while in the succession of 

 the former it becomes different." ^ 



I will now endeavour, by familiar illustrations, 

 to make clearer these apparently opposing theo- 

 ries. According to the view of those who believe 

 that acquired characteristics are transmitted, hu- 

 man life is like a clear stream which flows from 

 the mountains. On its way it passes through one 

 region after another in endless succession, re- 

 ceiving in its varied course something from a 

 thousand rills and rivulets on the surface and in 

 the soil, so that it is no longer pure as at first, but 

 carries with it many and varied substances from 

 many and varied districts. In other words, the 



1 Essays on Heredity, Weismann, pp. 105, 106. 



