818 THE DESCENT OF MAN 



classes. But this implies that the two sexes follow different 

 habits in their struggles for existence, which is a rare oir- 

 cumstance with the higher animals. The case, however, is 

 widely different with the reproductive functions, in which 

 respect the sexes necessarily differ. For variations in struc- 

 ture which are related to these functions have often proved 

 of value to one sex, and, from having arisen at a late period 

 of life, have been transmitted to one sex alone; and such 

 variations, thus preserved and transmitted, have given rise 

 to secondary sexual characters. 



In the following chapters I shall treat of the secondary 

 sexual characters in animals of all classes, and shall endeavor 

 in each case to apply the principles explained in the present 

 chapter. The lowest classes will detain us for a very short 

 time, but the higher animals, especially birds, must be treated 

 at considerable length. It should be borne in mind that, for 

 reasons already assigned, I intend to give only a few illus- 

 trative instances of the innumerable structures by the aid of 

 which the male finds the female, or, when found, holds her. 

 On the other hand, all structures and instincts by the aid of 

 which the male conquers other males, and by which he al- 

 lures or excites the female, will be fully discussed, as these 

 are in many ways the most interesting. 



Supplement on the Proportional Numbers of the Two Sexes 

 in Animals helonging to Various Glasses 



As no one, as far as I can discover, has paid attention to 

 the relative numbers of the two sexes throughout the animal 

 kingdom, I will here give such materials as I have been able 

 to collect, although they are extremely imperfect. They 

 consist in only a few instances of actual enumeration, and 

 the numbers are not very large. As the proportions are 

 known with certainty only in mankind, I will first give 

 them as a standard of comparison. 



Man. — In England during ten years (from 1857 to 1866) 

 the average number of children born alive yearly was 



