32 



The Proportion of the Sexes produced by Whites and Coloured 



Peoples in Cuba. 



By Walter Heape, M.A., F.R.S., Trinity College, Cambridge. 

 (Received September 30, — Eead November 26, 1908.) 

 (Abstract.) 



Introduction. — Darwin, in his great work on the Descent of Man, deals 

 with the proportion of the sexes in various animals and the power of natural 

 selection to regulate the proportional number of the sexes. He recognises a 

 general tendency to equality of the sexes but remarks on the fact that this 

 equality is often greatly disturbed. In certain rare cases of marked 

 inequality he concludes they might have been acquired through natural 

 selection, but in all ordinary cases, such as, for instance, the difference in the 

 proportion of the sexes in legitimate and illegitimate children, it can hardly 

 be so accounted for and must be attributed to unknown conditions, although, 

 he adds, natural selection will always tend to equalise the relative number of 

 the two sexes. 



About that time a host of writers were engaged in investigating various 

 possible causes for this inequality, and many theories were promulgated to 

 account for it, such as the relative age of the parents, the time of conception, 

 and so forth. Prominent amongst them was Diising, who set himself to show 

 that nutriment was the chief determining factor. He set forth his case with 

 great ability and brought an enormous mass of evidence in support of his view. 



Students of heredity in those days claimed that the laws of heredity were 

 sufficient to account for all inequalities, but Diising emphatically denied this, 

 and in my opinion satisfactorily showed he had sound reason for doing so. 



During the last few years much work has been done on sex, especially 

 regarding the factors which determine sex, and strong evidence has been 

 brought to show that both individual spermatozoa and ova are themselves of 

 definite sexuality. It is suggested that the sex of the individual resulting 

 from the conjugation of a spermatozoan and an ovum must be determined by 

 one or other of them, not by both, and it is claimed that, in order to fulfil the 

 conditions, a M. ovum must be fertilised by a F. spermatozoan and, vice versd, 

 a F. ovum by a M. spermatozoan. So far as the evidence available now goes, 

 it would seem possible that in some animals the sex of the offspring is 

 determined by the spermatozoan and in other animals by the ovum. The facts 

 are not clear, however, though it is to be hoped the efforts now being made by 



