224 GENERAL EVOLUTION. 



and dentine in shallow cayities represent the incisive dentition of 

 FrotolaMs. 



It remains to show that characters of the kind above men- 

 tioned are sometimes inconstant : that they may or may not ap- 

 pear in individuals of a species. Under such circumstances it is 

 evident that their origin does not imply any break in the line of 

 descent. 



First, as to a family character. It is well known that the deer 

 differ from the giraffes in the presence of a burr or ring of osseous 

 excrescences surrounding the base of the horn. Now, in the ex- 

 tinct tertiary genus Cosoryx there are three species which possess 

 or lack this burr indifferently. Why some individuals should, 

 and others should not possess it, is not known.* 



Second, as to a generic character. The genus Cams (dog) is 

 defined by the presence of two tubercular molars in the inferior 

 series. The allied genus Thous possesses three such teeth, while 

 Icticyon has but one. Now examples of Canis familiar is (domes- 

 tic dog) with but one tubercular molar are not rare, while an in- 

 dividual with three is occasionally found. 



To take another case. The normal dentition of Homo (man) 

 is, on each side, incisors, 2 ; canine, 1 ; premolars, 2 ; molars, 3, 

 It is very common to find in the higher races, individuals who 

 have molars only two in one or both jaws ; and the absence of 

 the external incisors of the upper jaw is almost as frequently met 

 with. Here we have two new generic variations in one and the 

 same species. 



In specific characters variations are most familiar. Thus, the 

 young of deer are generally spotted, and the adults are nearly 

 uniform in coloration. Some deer (as the Axis) retain the spotted 

 coloration throughout life, while an occasional spotted individual 

 of unicolor species is a violation of specific character by a failure 

 to develop. The larvae of some salamanders are of uniform col- 

 oration, and the adults spotted. The unicolor adults of the same 

 species, not uncommonly met with, present examples of the same 

 kind of variation. 



Any biologist can select hundreds of similar cases from his 

 special department of study. 



* The explanation I have offered is that the horn was stripped of its integuments 

 to the position of the burr by the animals in fighting. The condition of the speci- 

 mens renders this hypothesis probable. Sec "Report U. S. Expl. Surv. W. 100th 

 Mer.," Capt. G. M. Wheeler, vol. iv, pt. ii. 



