viii] HEREDITY 109 



variation ; but as has been seen in the case of hair- 

 colour the accurate investigation of individual families 

 would possibly show that several factors were con- 

 cerned, and that in the later generations, only when 

 all these factors are combined in one individual 

 would the colour be identical with that of either of 

 the original races. In this respect crosses between 

 different races of mankind resemble hybrids between 

 different species of animals or plants, except that 

 there is usually no sterility. Most of the Mendelian 

 investigations have been made on varieties which 

 differ in few characters, for the sake of simplicity, 

 but when species are crossed and the offspring are 

 fertile so many diverse characters are concerned, of 

 which the relation to one another is not generally 

 known, that the offspring of the hybrids may con- 

 tain no individuals closely resembling either parent 

 species. This has been explained by saying that only 

 varietal and not specific characters segregate from 

 one another on the Mendelian scheme, but it is not 

 improbably due to the multiplicity of characters con- 

 cerned, and their complicated interrelations, which 

 makes analysis exceedingly difficult. It is also 

 not impossible, when germ-cells differing very con- 

 siderably in constitution combine in fertilisation, 

 that in the formation of the germ-cells of the next 

 generation the machinery for segregation is inade- 

 quate. Extreme cases of this are possibly the cause 



