24 Plant Life and Evolution 



individual differences existing between any two 

 members of the same species. The germ cells, as a 

 rule, are so situated as to be less subject to external 

 influences, and are much more stable than the ordi- 

 nary vegetative or somatic cells. How far they may 

 be affected by external conditions, and to what ex- 

 tent, if any, changes thus affected are transmitted 

 by heredity, is one of the questions which has been 

 very much discussed, but about which there is really 

 very little positive evidence. 



ONTOGENY 



Ontogenetic Variations. Every organism passes 

 through a more or less extensive series of changes 

 during its development from the germ to maturity. 

 In the course of its life-history, or " ontogeny," in- 

 dividual variations occur, some of which can be 

 attributed to the environment, while others are ap- 

 parently innate. It is these ontogenetic variations 

 which distinguish the innumerable individuals be- 

 longing to a species. We may examine a hundred 

 seedlings, reared apparently under the same condi- 

 tions, and no two will be exactly alike. The biolo- 

 gist has no more difficult problem than that of 

 determining the causes of these variations. While 

 in many cases the effects of extrinsic factors can 

 be easily demonstrated, as for instance dwarfing 

 caused by insufficient nutrition, more often the varia- 

 tions seem to be innate differences which begin in 



