152 ON VARIABILITY AND ADAPTATION 



Following this line of thought, we can understand how it 

 comes to pass that the body cells and the germ cells in the 

 higher and more complex organisms become so sharply divided, 

 how it is that the body cells are no longer able to reproduce the 

 whole organism. Their idioplasm has become altered in certain 

 directions to such an extent that it is able, under favourable 

 conditions, to divide and reproduce cells of like nature, but the 

 very extent of the modification it has undergone has taken from 

 it that constitution or structure which is necessary to allow it 

 to reproduce each and every order of cells which together form 

 the individual. The germ cells, on the contrary, as a function 

 of their position in the organism, undergo no such extensive 

 changes in constitution, their idioplasm as it grows and is dis- 

 tributed into the successive germ cells retains its fundamental 

 constitution with but little alteration, and when these germ 

 cells are discharged they and their idioplasm, brought into like 

 relationships to those affecting the parent germ cells, undergo 

 the like series of developmental changes, and reproduce the 

 whole series of cells, tissues, and organs characteristic of the 

 species to which they belong. 



In the terms of this theory, therefore, inheritance essentially 

 depends upon the chemical constitution of the idioplasm or the 

 life-bearing or biophoric protoplasm of the germ cells, not upon 

 the number of the separate ids or biophores or ancestral plasms 

 or pangenes contained in the idioplasm ; and variation, whether 

 slight and individual, or extensive and leading to the production 

 of species, is ultimately the expression of modification in the 

 constitution of that idioplasm brought about by environment. 

 Whereas Weismann's theory lays stress upon relative fixity in 

 the constitution of the idioplasm, this theory admits freely the 

 capacity for change in structure of the same. So long as the 



has continued to grow under ordinary conditions without once developing 

 spores. By no means save altered environment is it possible to explain 

 Vincent's conversion of the absolutely harmless potato bacillus or, again, the 

 Bacillus megatherium (by long - continued sojourn within closed collodion 

 capsules in the peritoneal cavity of animals) into forms profoundly pathogenic, 

 and fatal to rabbits, mice, and guinea-pigs in the course of a few hours (Ann. 

 de Vlnst. Pasteur, xii. ? 1898, 785). 



The argument that phenomena observed in unicellular organisms cannot 

 be applied to multicellular organisms is, to say the least, a severely strained 

 argument. The extent to which environment acts as a factor may, it is true, 

 be diminished in the latter, but surely it cannot be regarded as being eliminated 

 and rendered negligible. 



