ACQUIRED CHARACTERS xlix 



limited to those that we may easily discern. We do not 

 at present know the determining factor and immediate 

 physico-chemical cause of growth and development. We 

 know only the mediate or more remote causes, such as 

 nutrition, increased blood-supply, etc. Hence it may quite 

 well turn out that the immediate cause of an acquired modifi- 

 cation is of the same nature as that which impresses on the 

 germ-plasm the tendency towards a specific variation. 



If the development of an individual is controlled by the 

 environmental factors, and if those factors are in the last 

 analysis of physico-chemical nature, then there seems reason 

 to suspect that the fundamental difference between a varia- 

 tion and a modification is not related to any difference in 

 their aetiology, or in the factors which produce them, but 

 is dependent almost wholly on the period of the individual's 

 life at which these factors operate. If they come into 

 action after birth or before it in the course of development, 

 they produce a modification apparently not heritable. If 

 they come into action before development begins, they 

 produce a variation which is heritable. 



Now, if we define the span of an individual's life as the 

 period contained between the moment of fertilisation of 

 the ovum and the moment of death, it is obvious that the 

 rate of development is exceedingly different at different 

 periods of this life. Take, for instance, a mammal. While 

 still in the uterus, it passes through every stage of develop- 

 ment from the protozoon to its own specific type. Before 

 it is even born, it has traversed with extreme rapidity and 

 many short-cuts the various stages passed through in the 

 course of past evolution by the species to which it belongs. 

 If therefore we judge of the maturity of an individual, not 

 by the time elapsed since fertilisation, but by the stage of 

 development attained at any given moment, it is clear that 

 a mammal, when born, has already passed through by far 

 the greater part of its life- journey : its career has already 

 reached the final stage, and verges on complete maturity. 

 The remainder of its independent career is doubtless, when 



