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 vazson ad’étre of this nuclear fusion appears 
to lie in the fact that the nucleus is the carrier of hereditary 
variations. Hence fertilisation 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 deczous, 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 grimary 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. 
