1 84 ESSENTIALS OF BIOLOGY 



phroditism (see Section 67) is about as common as that of separate 

 sexes. In practically all the plants this is the case and in all 

 groups of higher animals hermaphroditism in individuals may 

 occur. In most animals the male has rudimentary, or vestigial 

 female characters and vice versa. 



Mendelism and Sex. Sex can be regarded as an allelomorphic 

 character (see Section 80). That is maleness and femaleness 

 are mutually exclusive characters (but see hermaphroditism, 

 '' intersexes," " sex-mosaics," " gynandromorphs," etc.). A 

 female animal may carry " recessive " maleness and vice versa. 

 Sex may be '* linked " with other characters (Section 80). There 

 is evidence that of the spermatozoa produced by the individuals 

 of a race, some may be male-determining and others female- 

 determining. So also with the ova. These sex-conveying 

 potencies are regarded as inherent in the chromosomes of the ova 

 and spermatozoa and subject to reassortments at random (Section 

 80^). The hypotheses are rather complex and are mainly logical 

 ones. 



Nutrition and sex : sexual hormones. There is also evidence 

 (even in human populations) that sex ratios may be affected by 

 the nutrition of the eggs, or embryos, or pregnant mothers. 



There is plenty of evidence that the sex glands provide sub- 

 stances which circulate in the blood and may affect the nature 

 of the accessory, or unessential sex-characters. Castration, in 

 either sex, in many animals, has marked effects on bodily char- 

 acters (plumage, voice, etc.). There is even evidence that sex 

 may be reversed. It has long been known that old hens may 

 begin to " crow " as they cease to lay eggs. Even male sex glands 

 may develop in such animals. 



67. OAT HERMAPHRODITISM 

 Most of the higher plants have both male and female sex organs. 

 Most of the great groups of animals contain sub-groups that are 

 hermaphrodite and this condition may be present in large and 

 important groups of animals. Exceptionally hermaphroditism 

 occurs in practically all animals — even in man. 



In this condition, when it is typical, the same individual is both 

 male and female — that is, has functional ovaries and testes. 

 Functioning of both sex-glands may be simultaneous and copula- 



