160 Heredity. 



affect all the homologous parts of the offspring in the 

 same way and at the same time. 



Males more Variable than Females. 



One of the most remarkahle and suggestive of the laws 

 of variation is that in all the higher animals a part which 

 is confined to males, or is more developed or of more func- 

 tional importance in males than it is in females, is very 

 much more variahle than a part Avhich is confined to fe- 

 males or is more important in females than it is in males. 



The evidence for this remarkahle law will he presented 

 at length in Chapters VIII. and IX. The existence of 

 such a law is absolutely inexi)licable without the theory of 

 heredity, but it is exactly what this theory would lead us 

 to expect, for an organ which is most important in one 

 sex is most likely to be influenced in this sex by changed 

 conditions, and is therefore more likely to form gem- 

 mules in the body of the sex where it is most important 

 than in the body of the opposite sex. An organ which 

 is most important in males will therefore be most prolific 

 of gemmules in males, while an organ which is most im- 

 portant in females will be most prolific of gemmules in 

 females. Gemmules which are formed in the male body are 

 vastly more likely to be transmitter! to descendants than 

 those which are formed in the female body. It follows 

 that an organ which is most developed or most impor- 

 tant in males must be vastly more likely to transmit 

 gemmules to descendants, and therefore to vary in suc- 

 cessive generations than an organ which is most devel- 

 oped or most imj)ortant in females. 



Another law which follows from the one which has 

 just been stated is that males are as a rule more variable 

 than females. This law has been noticed by Darwin 

 and others, but no explanation has ever been advanced. 



