8 DEVELOPMENT AND HEREDITY. 



variations would become ever smaller and smaller 

 and eventually disappear. If the Metazoa separated 

 into groups, the same process would occur in each 

 group. Artificial selection has proven that where 

 only similar animals are allowed to interbreed, the 

 stock is kept pure and variations are not likely to 

 occur. 



Another objection to this theory of heredity — 

 and it seems to me insuperable — lies in the sup- 

 position that the germ-plasm may exist in the 

 body, undoubtedly a living part of it, and still be 

 no more affected by the changes which pass over 

 the body, than if it were enclosed in an hermeti- 

 cally sealed vial. This idea seems to be based on 

 a peculiar assumption in regard to the individuality 

 of a cell, as though the neighbouring cells of the 

 same organism were as distinct from each other 

 physiologically as they are morphologically ; or that 

 the cell walls are such firm and impermeable bar- 

 riers that the molecular condition of one cell 

 might be changed without affecting its neighbour. 

 We shall have occasion to consider this subject in 

 a later chapter. 



In accordance with this theory of variation and 

 heredity. Professor Weismann and his school deny 

 that the environment causes variations of growth for 

 the advantage of the organism. They believe in an 



