C. E. Beecher — Origin and Significance of Spines. 261 



determined the survival of the well-armed individuals, as in 

 the zoea of Cancer™ (figure 44), Carcinus, Ho?nams, etc. 



44. 



Figure 44. Zoea of the common crab, Cancer irroratus ; lateral view, x 8. 

 (After Yerrill and Smith 68 .) 



IV. Secondarily from sexual selection. (A 4 , B 4 .) 



The males and females of so many animals present dif- 

 ferences in size, color, and ornament, that corresponding varia- 

 tions 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 

 Ruminata, there are species without horns, other species with 

 horns in both sexes, and still others with horns only in the 

 male. In the wild state, the presence or absence of horns 

 and their character in any particular species seem to be well 

 established, but in domesticated forms, the greatest variety is 

 found. Among domesticated cattle, presumably of one species 



