NECESSITY OF USEFUL VARIATIONS 81 



for such, a development, then time was likewise available for 

 secondary variations and progressive adaptation of the other 

 parts — the muscles, nerves, bones, etc. The antlers themselves 

 probably varied but little in each generation, and there is no 

 reason why there should not have been similarly gradual pro- 

 gression in the development of the secondary variations in other 

 parts. If the development of the secondary variations did not 

 proceed step for step with that of the primary variation, it is 

 by no means necessary to assume that the individual would 

 thereby immediately be placed in a position of inferiority in the 

 struggle for existence. 



Useful variations, as we have said, are not only possible, but 

 necessary, if we grant the fundamental assumption that the germ- 

 plasm is composed of determinants. Therefore, whenever the 

 coadaptive strengthening of the muscles, nerves, etc., of a stag, 

 in harmony with the development of the antlers, became definitely 

 useful, we must suppose that the determinants of the muscles 

 and nerves varied in a progressive direction — that is to say, in 

 the direction of further development. The germ-plasm of 

 numerous stags — nay, even of the majority — may at first have 

 contained but a minority of ids with a majority of determinants 

 in the plus direction ; but those stags whose germ-plasm con- 

 tained a majority of ids with a majority of plus-varying deter- 

 minants would alone be favourably selected in the struggle for 

 existence. Thus there is an indirect connection between the 

 utility of a variation and its appearance. 



"We have not as yet sufficient knowledge of the details of 

 coadaptation to enable us to ofEer a satisfactory explanation 

 of these phenomena. We can, however, say with confidence 

 that no a priori objection can be raised against their explanation 

 by natural selection. And though we have as yet insufficient 

 data to enable us to explain all the facts of coadaptation of 

 active parts, we should remember that coadaptation is not con- 

 fined to active parts, but is also observable in passive parts ; 



6 



