74 



KANSAS CITY REVIEW OF SCIENCE. 



1,1 .ji 



From these facts it is obvious that the Hippotheroid line terminated in Equus 

 while the Protohippoid division terminated in Hippidium, and is extinct. 



I have now made clear, I trust, the successive steps in modi- 

 fication of limb and dental structure from the five-toed semi- 

 plantigrade bunadont Phenacodus to the one toed digitigrade 

 selenodont Equus as exhibited in time by the different geologic 

 horizons. It now remains to inquire into the causes that have 

 led to these changes. Have they been the result of natural or 

 physical forces, that are in operation around us to day, as well 

 as in the remote past, or have they been produced by supernat- 

 ural influence in successive creations, as opposed to the conti- 

 nuity of natural law, and by methods unknown ? If this latter 

 proposition be true we should find important breaks, sharply de- 

 fining each new creation in the line of succession. But as I 

 have attempted to show, the transition has been a gradual one 

 and that each succeeding form in the ancestral line is distin- 

 guished only by the intensity of its modification in a given di- 

 rection, the weight of opinion must rest with the former. I 

 dare say that if all the intervening individuals between Phena- 

 codus and Equus could be produced classification would be ut 

 terly impossible, so insensible would be the gradation. Consid- 

 ering the unfavorable conditions for their preservation, and the 

 fractional part possible to examine in comparison to the whole 

 area of deposit, the most remarkable fact is that palaeontology 

 Left fore foot of can present such an array of evidence as it does. 

 ^th''narti2f ^(a°ft"et When we speak of physical methods much is implied ; but 



Gaudry). if wc scck for adcquatc causes to explain these changes, no field 



offers more conclusive results than the ordinary me- 

 chanical ilses of certain organs, as affecting their 

 metamorphoses. In regard to tooth structure gen- 

 erally, Mr. J. A. Ryder has given us a most excel- 

 lent treatise, "On the Mechanical Genesis of Tooth 

 forms,"* in which he shows that the jaw movements 

 of animals are intimately related to the modifica 

 tion of the component lobes, crests, and ridges of 

 the crowns of the molar teeth. He also points out 

 that the restricted jaw- movements in which the 

 mouth was simply opened and closed, is associated 

 with the bunadont molar. That the various kinds 

 of excursive mandibular movements have been de- -^?a«^ / natural size, 

 veloped progressively. " That as these excursive movements have increased in 

 complexity there has been increase in the complexity of the enamel foldings." 



Fig. 22. 



ace 



pi 



Fig. 23. 



Left superior molar of a 



species of 



• Proceedings Academy Natural Sciences, Philadelphia, 1878. 



