126 INTRODUCTION TO SEXUAL PHYSIOLOGY 



will be formed likewise in equal numbers in accordance with 

 Mendelian expectation. 



It is noteworthy that the genetic evidence derived from the 

 study of sex-linked characters is confirmatory of the hypothesis 

 just outlined, for it is believed that some characters, like the 

 orange colour of certain male cats or the tortoiseshell colour of 

 female cats, are located in the sex chromosomes. It only very 

 occasionally happens that the linkage is broken, as when tortoise- 

 shell males are produced, and it is noteworthy that these animals, 

 although possessing testes, are sterile. 



In the case of birds and Lepidoptera, in which the female 

 is heterozygous for sex, the spermatozoon ahvays has one X- 

 chromosome, while half the ova are without an X-chromosome. 



AVe have seen that the approximate numerical equality of the 

 sexes which is the rule among animals is explained as due to the 

 equal production of male- and female-producing spermatozoa 

 (or ova). The slight divergencies from numerical equality between 

 the sexes which occur from time to time and place to place are 

 caused by subsidiary factors. Since in mammals appreciably 

 more mates are conceived than females, it is to be supposed 

 that the balance between male-producing and female-producing 

 spermatozoa is upset at or before fertilisation. The excess of 

 males, however, is considerably reduced during gestation by the 

 high mortality among the males (as show^n by Parkes). This 

 equality mechanism, which we must regard as j^rimitive, having 

 become established, is maintained even among polygamous 

 animals where it has ceased to be useful, since one male is the 

 physiological equivalent of a large number of females, a fact 

 which is taken advantage of by man, who castrates by far the 

 larger number of males among domestic animals for economic 

 reasons, keeping only the best for stud purposes. This superfluity 

 of males may, however, have favoured the conditions under which 

 preferential mating has been supposed to operate, as postulated 

 by Darwin's well-known theory of Sexual Selection. This theory 

 was propounded to supplement the theory of Natural Selection 

 or the survival of the fittest, and to account especially for those 

 secondary sexual characters, usually of the natm'e of embellish- 

 ments, such as the wonderful colouring of many birds and insects, 

 the bird's power of song, the tail of the peacock and pheasant, 

 and that of the bird of paradise, as well as the antlers of the 



