1892.] NATURAL SCIENCES OP^ PHILAUELPHIA. 409 



mil- gegebenen Figuren liisst eine solclie Auifassung direct als hin- 

 fiillig erscheinen." 



To these criticisms it may be replied : (1 ) It is perfectly true tliat the 

 present state of paljeontological knowledge does not permit the con- 

 struction of an undoubted phyletic series from the Triassic to the 

 Puerco, but when the geological and the morphological succession agree 

 so well, we are entitled to assume, in the absence of any evidence to 

 the contrary, that the steps of the change are those indicated by 

 the successive genera of mammals. Nobody has stated that these 

 steps were of necessity those which the evidence at present avail- 

 able appears to show ; on the contrary, Osborn has expressly 

 pointed out another possible method. 



(2) The relations of the "entire organization of the animal 

 world of the Puerco times " to that of the present, are, it is true, far 

 from clear, and yet the connection of many Puerco genera with 

 their Wasatch successors is so obvious as to deprive this objection 

 of any Aveight. The hypothesis that all the varieties of mammalian 

 molars were derived from a single type of tooth is much more than 

 a "seductive speculation." For many different groups it has such 

 a high degree of probability as almost amounts to demonstration, 

 for no group has it been shown to be untrue, and in those in which 

 we cannot demonstrate its truth, the necessary phyletic series has 

 not yet been discovered. On the other hand, the history of the 

 pi'emolars makes it evident that similar stages of dental evolution 

 may be reached in different ways. 



(3) It is no objection to the truth of a morphological fact that 

 we cannot give a physiological explanation of it. Indeed, the 

 lectors of transformation form at present the most actively disputed 

 questions of biology. But whether or not mechanical (or dyna- 

 mical) factors of change are admitted as efficient causes of trans- 

 formation, it is clear that they at least indirectly condition the 

 result and that mechanically unfavorable changes cannot be per- 

 petuated. It is not difficult, in a general way, to see why aniso- 

 gnathism and movements of the jaw other than vertical should 

 necessitate the reversed arrangement of points upon the crowns of 

 the upper and lower molars, because they are subjected to opposite 

 strains. Even in the premolars, where the homologous cones are 

 not reversed, the reversal of form is as clearly shown as in the 

 molars, i. e. when the premolars reach a stage of complication 

 comparable to that of the molars. 



