522 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



tation is increased by local and general metatrophic 

 changes of necessity correlated, which take place most 

 rapidly in the regions of least perfect adaptation, since 

 here the reactions are greatest ; the main term of vari- 

 ation is determined by the slow transmission, not of 

 the full increase of adaptation, but of the disposition 

 to adaptive atrophy or hypertrophy at certain points ; 

 the variations thus transmitted are accumulated by the 

 selection of the individuals in which they are most 

 marked, and by the extinction of inadaptive varieties 

 or species ; selection is thus of the ensemble of new 

 and modified characters. Finally, there is sufficient 

 paleontological and morphological evidence that ac- 

 quired characters in the above limited sense, are trans- 

 mitted. . . . Excepting in two or three side-lines the 

 teeth of all the Mammalia have passed through closely 

 parallel early stages of evolution, enabling us to formu- 

 late a law : The new main elements of the crown make 

 their appearance at the first points of contact, and chief 

 points of wear of the teeth in preceding periods. What- 

 ever may be true of spontaneous variations in other 

 parts of the organism, these new cusps arise in the 

 perfectly definite lines of growth. . . . Now, after ob- 

 serving these principles operating in the teeth, look at 

 the question enlarged by the evolution of parallel spe- 

 cies of the horse series in America and Europe, and 

 add to the development of the teeth what is observed 

 in progress in the feet. Here is the problem of corre- 

 lation in a stronger form even than that presented by 

 Spencer and Romanes. To vary the mode of state- 

 ment : what must be assumed in the strict application 

 of the selection-theory? (a) that variations in the lower 

 molars correlated with coincident variations of reversed 

 patterns in the upper molars, these with metamorpho- 



