44 SECONDARY SEXUAL CHARACTERS. 



or difference in structure between the sexual elements, 

 apparently confined to the protoplasm, promotes and ensures 

 the fusion of elements from two separate individuals. The 

 nutritive conditions of the male and female, with a deficiency 

 and an excessive proportion of protoplasm respectively, con- 

 duce to their mutual fusion and prevent fusion of the same 

 elements. The raisoii d'etre of this nuclear fusion appears 

 to lie in the fact that the nucleus is the carrier of hereditary 

 variations. Hence fertiHsation ensures that every cell of the 

 new individual shall partake of the characters of at least two 

 antecedent organisms. This can only be effected by every 

 organism starting as a single cell. 



As quite a secondary phenomenon we have what is 

 called dimorphism of the sexes. We have seen that the 

 sexual elements differ, the male element being the active 

 agent in reaching the female element, which itself is passive. 

 This physiological division of labour and consequent struc- 

 tural dissimilarity between the sexual elements is, in many 

 higher animals, reflected back to the reproductive organs of 

 the parent, producing male and female individuals or sexes. 



If the sexual organs are found in separate individuals 

 the species is called diceeioiis, if united in one individual the 

 species is described as hermaphrodite. Hermaphroditism is 

 of widespread occurrence throughout the animal kingdom, 

 but is rare in higher types. 



In certain exceptional instances the female may produce 

 eggs which, without fertilisation, may develop into fresh 

 individuals. Such a phenomenon is termed parthenogenesis. 



As a general rule, the sexual organs are the last to mature ; 

 hence the reproductive function only takes place after all 

 development has ceased, but in certain rare instances a 

 larval form is known to attain maturity and reproduce itself. 

 The phenomenon is known as pcedogenesis. Axolotl is a 

 good example. 



The differences in the structure and function of the 

 sexual organs are called primary sexual characters, but in 

 those animals in which the fertilisation is not promiscuous 

 the sexes often show structural differences other than those 

 of the sexual organs. As examples we may cite the plum- 

 age of birds and the antlers of deer. These are called 

 secondary sexual characters. 



