ORIGIN AND SIGNIFICANCE OF SPINES 57 



Another process or kind of selection has been described by 

 Verrill as " Cannibalistic Selection." He has shown that 

 the young of carnivorous animals often prey upon each 

 other, as in the larval forms of some Decapoda, or sometimes 

 even before the escajje of the young from the egg capsules, 

 as in some of the Gastropoda. Here, of course, any natural 

 ^'ariation in the newly hatched animals which would give an 

 individual some advantage over its companions would tend 

 to its piescrvation and to their destruction. In this way it 

 may occur that the relative growth of spines in the zoea of 

 decapods has determined the survival of the well-armed 

 individuals; as in the zoea of Cancer ^'^ (figure 44), Carcinus, 

 llomarus, etc. 



IV. Secondarily from sexual selection. (A^, B4.) 



The males and females of so many animals present dif- 

 ferences in size, color, and ornament, that corresponding 

 variations in the development of spines, horns, and antlers, 

 might naturally be expected. That such differences actually 

 occur in nature is evident. Every gradation can be found 

 between horns or antlers common to both sexes and those 

 confined to one sex. Probably the initial difference is as 

 ancient as sex itself. 



Sexual variations of horns are most familiar among the 

 mammals. Some, as the Giraffe, Ox, Bison, and Reindeer, 

 have them present in both sexes, though the antlers of the 

 female Reindeer are smaller and more slender than in the 

 male, and in the American variety are sometimes absent. 

 Others, as in the Prong-horn Antelope, many sheep, goats, 

 etc., have the horns usually quite small in the female, and 

 well developed in the male. Lastly, the modern Deer, Elk, 

 Moose, etc., have the antlers confined to the males alone, the 

 female being entirely without them. 



Some of the early deer (Procervulus) seem to have had 

 antlers in both sexes, and in nearly all the families of the 



