364 T^he Theory of Natural Selection 



so much additional value that the individuals possessing it 

 survived to the exclusion of the kallimas who looked like 

 dead leaves without this stigma of decay. 



We can see, granting the premises, how natural selec- 

 tion would lead to stronger and sharper weapons of offense 

 and defense. But we cannot see how this same process would 

 lead to the development of tusks of such form and magni- 

 tude that they would interfere with the possessor's use of 

 them, or to the development of horns that were too heavy 

 for the animal's muscles to manage. Other examples of 

 overspecialization are to be found both among existing 

 forms and among those that have become extinct (see 

 page 189). Whatever a given structure, pattern, or be- 

 havior may contribute to the survival of a species, we cannot 

 assume it to increase in value past a certain point. In fact, 

 the indications are that such overspecialization is injurious 

 to the species. There is a point beyond which good things 

 cease to be of virtue. 



Sexual Selection 



Among many animals, especially birds and mammals 

 and some insects and other arthropods, there is a constant 

 difference between the two sexes in body form and in special 

 organs. The song of the bird, the mane of the lion, the 

 horns of the deer are familiar examples ^f these secondary 

 sexual characters confined to the male. 



Recent experimental studies have connected the de- 

 velopment of many such structures and characters with the 

 internal secretion of gonads (the ovaries and the spermaries) . 

 A castrated male will fail to develop the distinctive charac- 

 ters of his sex. If a spayed female has implanted in the body 

 the spermary from the male of the same species, the female 

 will develop some of the characteristics of the male. These 

 facts had not been so clearly perceived in Darwin's time; 

 and the theory of natural selection had to explain these 

 sex differences (see opposite page). 



