I. 



Factors in the Evolution of the Mammalia. 



AT the close of his valuable memoir " On the Osteology oiMesoJiippits 

 and Leptomeyyx " in the last number of the Journal of Morphology 

 (Vol. v., No. 3) Dr. W. B. Scott, of Princeton College, New Jersey, 

 devotes a section to a critical consideration of some of the factors in 

 the evolution of the mammalia. Therein, at the outset, he expresses 

 his dissent on general grounds from Dr. August Weismann's now 

 familiar theory of the continuity of the germ-plasm, according to 

 which it is impossible for any character or modification of structure 

 acquired during the life of the parent organism to be transmitted to 

 the offspring which that parent subsequently produces. Quoting from 

 Mr. W. H. Dall, he says : " The contention of Weismann, that 'not 

 a single fact hitherto brought forward can be accepted as proof of 

 the transmission of acquired characters , . . reminds one of the 

 familiar statement of twenty years ago, that the Darwinians had 

 not brought forward a single instance of the conversion of one species 

 into another species. If the Dynamic Evolutionist brings forward an 

 hypothesis which explains the facts of nature without violence to sound 

 reasoning, that hypothesis is entitled to respect and consideration until 

 some better one is proposed or some vitiating error detected in it." 



The special object of Dr. Scott's discussion is to show that the 

 careful examination of certain series of fossil mammals indicates 

 clearly that the Dynamic theory, that is to say, the hypothesis of the 

 inheritance of acquired modifications of structure due to the direct 

 action of the environment, is more probable than the hypothesis of 

 unaided Natural Selection ; or, to state the contention in his own 

 words, " that transformation, whether in the way of the addition of 

 new parts or the reduction of those already present, acts just as if 

 the direct action of the environment and the habits of the animal 

 were the efficient cause of the change, and any explanation which 

 excludes the direct action of such agencies is confronted by the 

 difficulty of an immense number of the most striking coincidences." 

 The evidences in favour of this contention are briefly as follows : — 

 (i.) The changes which are observed to occur in the fossil 

 mammals under consideration are carried out in exact accord with the 

 mechanical exigencies of the case. New facets on the bones and 

 new cusps on the teeth appear in definite ways and in definite places. 

 In the structure of the carpus and tarsus, the bones which answer to 



