386 SCOTT. [Vol. V. 



We have seen that the proximal end of the humerus is very 

 similar in the White River representatives of the horses and 

 camels (Mesohippus and Pcebrothermm), having the bicipital 

 groove single, narrow, and placed at the antero-internal angle 

 of the head, while the external tuberosity is very large, and ex- 

 tends across nearly the entire anterior face of the bone. In 

 the modern forms the bicipital groove is double, being divided 

 by the large bicipital tubercle, and the external tuberosity is 

 reduced to the size of the internal one. If all of these changes, 

 which are carried out in such exact accord with the mechanical 

 exigencies of each case, are not somatogenic, how can they be 

 accounted for ? What chance is there of such occurrences 

 repeatedly taking place in widely separated groups of animals 

 as the result of sexual reproduction, or of changes in the germ- 

 plasm which stand in no other causal connection with the 

 mechanical needs of the body, than that those variations which 

 happen to be favorable are preserved ? The improbability of 

 such an explanation is still further increased by the fact that 

 the numerous non-mechanical variations, which on this theory 

 ought to occur, are not to be found in the fossil series except 

 very rarely, and then as the manifest results of disease or 

 accident. Why should there be such a profound difference 

 between the method in which useful changes are brought about 

 in the individual and those which modify the species ? and how 

 can we explain the mysterious pre-established harmony between 

 the two classes of phenomena ? It seems very wonderful that an 

 "ever-vigilant natural selection," which introduced sexuality for 

 the very purpose of producing and combining variations, should 

 have failed to seize and make use of somatogenic changes. 

 This class of phenomena is by no means confined to the 

 vertebrate skeleton. Dall has shown how the characters of the 

 hinge and shell of the bivalve mollusks have been evolved in 

 accordance with mechanical principles (No. 12, p. 445). He 

 has also shown " how the initiation and development of the 

 columellar plaits in Voluta, Mitra, and other Gasteropods, is 

 the necessary mechanical result of certain comparatively simple 

 physical conditions ; and that the variations and peculiarities 

 connected with those plaits perfectly harmonize with the results 

 which follow with inorganic material subjected to analogous 

 stresses" (No. 13, p. 9). Neumayr, in describing the transition 



