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- 
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