1008 THE PHYSIOLOGY OF REPRODUCTION. 



The results obtained from crossing hybrids is not always as simple 

 as is indicated in this schema, since so-called characteristics may 

 be dependent not upon a single but upon several factors. In 

 general, however, the Mendelian principles of the existence of unit 

 characters and segregation of these characters in the germ cells are 

 accepted. They throw some hght upon what was before a very 

 mysterious process. The point of view suggested by these principles 

 leads us to lay great stress, in the matter of breeding, upon the im- 

 portance of the characteristics of the racial strains, and to value 

 less than formerly the importance of characteristics acquired during 

 the life of the individual parent, or, expressed in another way, the 

 modern tendency is to believe that "nature counts for more than 

 nurture." As a matter of fact in the development of an individual 

 both nature and nurture play important parts; nature, that is the 

 character of the stock, determines the potential possibilities in the 

 fertihzed ovum, while nurture, or the character of the environment, 

 is influential in exciting or repressing the development of these 

 possibilities. The full comprehension of the fundamental import- 

 ance of the heredity factor is mainly responsible for the modern 

 movement of eugenics, which hopes to improve the quality of the 

 race by limiting the growth of poor stock in mankind and encour- 

 aging the breeding from the best stock. 



Determination of Sex. — The conditions which lead to the 

 determination of the sex of the developing ovum have attracted 

 much investigation and speculation In the absence of precise 

 data very numerous and oftentimes very peculiar theories have 

 been advanced.* Such views as the following have been main- 

 tained: that the sex is determined by the ova alone; that it is 

 determined by the spermatozoa alone; that one side (right ovary 

 or testis) contains male elements, the other female; that the sex 

 is a result of the interaction of the ovum and spermatozoon, the 

 most virile element producing its own sex, or according to another 

 possibility "the superior parent produces the opposite sex"; that 

 the sex depends on the time relation of coitus to menstruation, 

 fertilization before menstruation favoring male births, after men- 

 struation female births; that it depends upon the nutritive con- 

 ditions of the ovum during development or of the maternal parent; 

 that it depends upon the relative ages of the parents; that there 

 are preformed male and female ova and male and female sper- 

 matozoa, etc. What we may call the scientific study of the problem 

 began with the collection of statistics of births. Statistics in Europe 

 of 5,935,000 births indicate that 106 male children are born to 



* For accounts of the various theories and discussion, see Morgan, "He- 

 redity and Sex," 1913; Wilson, "Proceedings of the Roy. Soc," B, 88, 1914 

 (Croonian Lecture); Lenhossek, "Das Problem der geschlechtsbestimmenden 

 Ursachen," 1908. 



