548 GENETICS IN RELATION TO AGRICULTURE 



plete data are published, but we may again emphasize the fact that 

 nothing in a disturbed sex-ratio need necessarily be taken to mean that 

 the mode of sex-determination is anything other than that which we 

 have stated in preceding portions of this chapter. 



Secondary Sexual Characters. By a secondary sexual character 

 is meant a character not immediately concerned with reproduction, 

 but found only in one sex. In the more highly organized animals the 

 differences between the secondary sexual characters of male and female 

 are so great that by means of them alone it is possible to recognize in- 

 stantly the sex of the individual. The secondary sexual characters 

 include a wide variety of characters some of which are very definite and 

 others are indefinite. Thus in most animals there are differences in 

 size and general conformation, for example the stallion is larger and 

 more rugged in build than the mare. The neck and forequarters are 

 more fully developed than in the mare. These are differences of degree 

 rather than kind. In certain breeds of sheep, however, horns are present 

 in the males and absent in the females. In birds the differences between 

 the sexes are often very striking, extreme instances of which are found 

 among domesticated birds in such breeds as the Brown Leghorn fowl and 

 the Rouen duck, and breeds of similar plumage coloration. 



The Nature of Secondary Sexual Characters. Although secondary 

 sexual characters are intimately related in expression to sex, yet careful 

 distinction must be drawn between the factor basis of secondary sexual 

 characters and that of sex-determination and sex-linkage. Sex-linked 

 characters, of course, are those which are determined by factors borne 

 by the sex-chromosomes. Such characters display peculiarities in he- 

 redity which are dependent upon the chromosome relations, but the 

 characters themselves may appear in either sex. Sex-determining 

 factors are those which determine sex. Their presence results in profound 

 effects upon the total developmental processes of the body. The different 

 internal physiological conditions, therefore, which exist in the two sexes 

 profoundly affect the reactions which the normal hereditary system 

 exhibits in consequence of which many characters in the two sexes 

 are different. These are the secondary sexual characters. In their 

 development no difference need be postulated in the factorial basis 

 save in the sex-factor itself. But the incentive to their development 

 is found in the different internal conditions in the two sexes; the factors 

 may be the same in both cases but these differences in internal condition 

 lead to differences in the reaction products of those factors. Evidence 

 of this view has been obtained largely from castration experiments. 



The Effects of Castration. The effects of castration differ so much 

 in different groups of animals that no general statement can be made 

 which describes all these results. Among mammals the effects of cas- 



