4 o 4 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



primitive Glires. The removal of the enamel from the 

 apices of the tubercles and crests of their descendents 

 is due to the abrasion consequent on ordinary use. 

 On this Ryder (/. <:.) remarks : " The great value which 

 is to be attached to the fact that abrasions of the 

 enamel of the adult, which have reacted upon the 

 functional activity of the enamel organ of the embryo 

 rat, so that such mechanically induced alterations could 

 be inherited, does not consist so much in the proof it 

 affords that mutilations can be inherited, as it does 

 that mutilations incurred in the ordinary struggle for 

 existence, may, under certain conditions in certain 

 practically feral species, be transmitted." 



Having shown by these examples that acquired 

 characters can be inherited, I offer some other illus- 

 trations which are at hand. 



b. Arthropoda. 



It has been already rendered probable if not certain 

 (p. 268) that the segments of the body and limbs of the 

 Arthropoda were originally produced by the movements 

 of definite tracts on each other, during the period that 

 the external surfaces were becoming hardened by chi- 

 tinous or calcareous deposits. It is well known that this 

 segmentation is no longer produced by this mechanical 

 cause during the adolescent or any other post-embry- 

 onic stage of the life of the individual, but that it ap- 

 pears during the various stages of embryonic life, and 

 is therefore inherited. Thus segmentation of the body 

 appears in the embryo while still attached to the yolk. 

 During the larval life of many insects the process of 

 segmentation is suspended, but during the repose of 

 pupal life, it goes on with great rapidity. In this stage 

 while protected from external mechanical stimuli, the 



