INTRODUCTION 9 



The bearing of these laws on the phenomena of colour 

 variation has been studied in many plants and animals, and 

 there is little doubt they will be of great value in the study 

 of the heredity of disease as well. As to what influence 

 these laws may have had on the evolution of the forms of 

 teeth, we are quite without evidence. 



Other factors have also to be considered, as sexual selection 

 and concomitant variation. 



Sexual selection is illustrated by the fact that those 

 individuals among the males which possess certain physical 

 advantages over their fellows, will be able to obtain the 

 mastery in their fights for the possession of the females, as 

 by the provision of stronger and better-developed horns 

 or tusks ; and their superior physical development would 

 be inherited. 



Concomitant variation or correlation of growth is shown 

 in the development of one organ at the expense of another 

 an animal with very large horns having suppressed canines, 

 and one with greatly developed tusks showing absence or 

 diminution of horns. 



One illustration of the truth of the doctrine of evolution 

 is the existence, in the course of development of the higher 

 forms, of vestigial remains of organs constantly present in 

 the adult forms of early progenitors^ as instanced by Darwin 

 in the teeth of Ungulates (36): 



' The calf has inherited teeth, which never cut through 

 the gums of the upper jaw, from an early progenitor having 

 well-developed teeth ; and we may believe that the teeth 

 in the mature animal were formerly reduced by disuse, 

 owing to the tongue and palate, or lips, having become 

 excellently fitted through natural selection to browse with- 

 out their aid ; whereas in the calf the teeth have been left 

 unaffected, and on the principle of inheritance at corre- 

 sponding ages have been inherited from a remote period to 

 the present day. 



* Disuse, aided sometimes by natural selection, will often 

 have reduced organs when rendered useless under changed 

 habits or conditions of life ; and we can understand on this 

 view the existence of rudimentary organs.' 



Darwin established the great principle of continuity 



