The Making of Species 
explanation of all the peculiarities of animal 
structure and colouration. 
It is not easy to understand how natural selec- 
tion can have caused marked sexual dimorphism 
in a species where the habits of the sexes are 
the same, in the Paradise Flycatcher ( Zexpszphone 
paradisc), for example, where the cock and the 
hen obtain their food in the same way, and share 
equally the duties of nest-building, incubation, and 
feeding the young. 
Of course, in all species where each individual 
carries only one of the two kinds of sexual organs, 
there must of necessity be some slight difference 
between the individuals that carry the male organ, 
which performs one function, and those that carry 
the female organ, which performs another function. 
But in many species the sexes display differ- 
ences which have no direct connection with the 
generative organs—for example, the deer, where 
the stag alone has horns. 
Those characters which differ with the sex, 
but are not directly connected with the organs 
of reproduction, are known as secondary sexual 
characters. 
In nearly all species where the male and 
female differ in beauty, it is the male who 
surpasses the female. Natural selection is, 
in many cases, not able to explain the origin 
of these differences, or why, when they occur, 
the male should be more beautiful than the 
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