gg NATURAL SCIENCE. 



April, 



those in the wrist and ankle of man, " we find that in any given 

 phylum [or series in direct descent] very definite lines of evolution 

 are early established and closely adhered to, and the changes are just 

 those called for by the operation of dynamical influences." More- 

 over, in the matter of modifications of tooth structure, in all cases 

 known to the author, the new points appear in places where, at some 

 stage, there is special abrasion. 



(2.) The evidence afforded by the reduction and loss of structures 

 supports that afforded by the growth and development of structures. 

 Such loss and reduction is ascribed by Dr. Weismann to " panmixia," 

 or the cessation of that selective process which raised them to a high 

 level of perfection and efficiency, and, so long as it continues, keeps 

 them there. When the selection ceases, those individuals which have 

 these now useless structures poorly developed are no longer weeded 

 out and, breeding with those individuals which have the structures 

 well developed, lower step by step the standard of efficiency. To 

 this explanation Dr. Scott dissents on general grounds ; and, with 

 respect to the special field of mammalian palaeontology, he says : 

 " When we turn to the series of fossils, and follow out the history of 

 disappearing organs, we find little to support the theory of panmixia. 

 The reduction is steady and sure, if slow, exhibiting, of course, a 

 certain degree of individual variation, but not the fluctuations which 

 we should naturally expect to find were panmixia alone the cause of 

 the reduction. As a matter of fact, when examining an extensive 

 series of fossils reaching through many horizons, it is difficult to 

 escape the suspicion that individual variations are not the material 

 with which Natural Selection works, so steadily does the series 

 advance toward what seems almost like a predetermined goal." 



(3.) While the development of useful structures and the loss or 

 reduction of useless structures are thus in close accord with the 

 mechanical conditions, there is no evidence of tentative trials and 

 false starts before the proper line of development is hit upon. If 

 variations in accordance with mechanical conditions were selected 

 from among other variations, we should expect to find evidence of 

 such other non-adaptive variations. These may, however, be sought 

 for in vain. The development of the carpus and tarsus may be 

 traced step by step in the equine series from Hyracotherium to Eqmis. 

 We find a definite chain of adaptive modifications. Non-mechanical 

 variations do not occur. 



(4.) Under the influence of similar environing conditions similar 

 modifications have been induced in parallel series of animals not 

 closely related to each other by genetic affinity. Thus, in several 

 different groups there appear suddenly and simultaneously (using 

 these terms in their geological sense) prismatic or rootless molar 

 teeth, with very complex enamel foldings, the valleys between the 

 harder enamel ridges being filled with the softer " cement." This 

 development of complex grinding teeth is found to be contemporaneous 



