CHAPTER VIII 



Causes of variation Amphimixis Not compatible with facts of maturation and 

 fertilisation Elimination of useless variations by bi-parental reproduction 

 Conclusions. 



MUCH has been written in explanation of the probable causes 

 of variation. A very favourite theory is that variations are 

 stimulated by the mingling of the germ plasm of two indi- 

 viduals, which always takes place in those organisms where 

 bi-parental reproduction is the rule. This mingling of the 

 germplasm, and the complicated distribution of the chromo- 

 somes in fertilisation, has been called " amphimixis," and it 

 has been put forward by many as the immediate cause of 

 variations in the offspring. Weismann was the first to 

 advocate this theory, and it has been supported since by 

 many others. 1 Now we have seen that individual chromo- 

 somes cannot possibly be the bearers of different racial 

 characters, but there seems nothing against their being the 

 bearers of individual characters. We shall consider later 

 whether it is probable that they are so ; at present it is 

 necessary to consider only what would be the effect of the 

 mode of distribution and recombination of the chromosomes 

 involved in fertilisation. We have seen that in the case of 

 man, where the cells contain thirty-two chromosomes, there 

 is only one chance in over six hundred millions of the same 

 combination occurring as was present in the parents, and 

 this is after making the assumption that the chromosomes 

 in the two parents are similar. At first sight this suggests a 

 possibility of innumerable variations, but when we come to 



1 Thomson, Heredity, p. 102 et seq., 1908; Ziegler, H. E., "Die Chromo- 

 somen-Theorie der Vererbung in ihrer Anwendung auf den Menschen," Archiv 

 Jlassen-gesellschaftliche Biologic, vol. iii. pp. 797-812. 



