8o The Third and Fourth Generation 



male sn+i. Each egg would have n-\-i chro- 

 mosomes. The sperm wiU be of two sorts, 

 those containing n chromosomes and those 

 having n-\-i. If an egg is fertilized by the 

 latter sort of sperm it will produce a female 

 (2W+2); if by the former a male (2^+1)- 

 Since the chances are equal that a sperm of one 

 sort will find eggs to fertilize as often as will 

 those of the other type, the number of males 

 ordinarily equals the number of females. In 

 some animals in which one sex greatly out- 

 numbers the other it has been shown that there 

 is a very high mortality among the sperm of one 

 type. 



Biologists are not agreed that the odd 

 chromosome is really the sex-determiner; it 

 may merely be a sex-indicator. But at any 

 rate there is established a specific relation 

 between a very conspicuous character, sex, and 

 the presence of a particular chromosome (or, at 

 times, group of chromosomes). This relation 

 has been found to obtain in a very great 

 variety of animals. Recent work tends to show 

 that the original impulse toward one or the 



