218 Life and Death, Heredity and Evolution 



into slightly differing stocks, similar to those found in na- 

 ture. The changes are either continuous, or by steps so 

 slight that single ones are hardly detectible. 



In the higher organisms, where the matter is greatly com- 

 plicated by the fact that most reproduction is from two 

 parents, it first appeared that gradual or minute hereditary 

 changes did not occur. But as we have tried to bring out 

 above, with more thorough study it has come to light that 

 such changes do occur, as well as do hereditary alterations, 

 which, when they reach the adult condition, form a sudden 

 marked change. In this way all contrast in principle be- 

 tween what we find in the lower organisms and what we find 

 in the higher ones disappears. In both the process of evo- 

 lution by minute gradations is visible. 



Then we proceeded to examine how far in lower organ- 

 isms diversity of external conditions brings about hereditary 

 changes. Here again the first examination, even though 

 long continued, seemed to show constancy; diversity of ex- 

 ternal conditions appeared to have no permanent effect on 

 the stocks. But again, intensified study reveals that the 

 hereditary characters gradually do become changed by di- 

 versities of external conditions. Through such diversities, 

 continuing for great numbers of generations, single stocks, 

 uniform in their hereditary characters, gradually differen- 

 tiate into many with faintly differing hereditary features. 

 Again the process is gradual, or by steps so small that sin- 

 gle ones are imperceptible. In higher organisms the state 

 of knowledge on this point appears less satisfactory. But 

 the evidence so far as it goes indicates that the processes 

 here are in agreement with those in lower organisms. Ap- 

 parently diversities in external agents may, under condi- 

 tions which seem rather rarely met, so modify the germ cells 

 that they produce progeny with changed hereditary char- 



