1072 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [92] 



were more or less permanently flexible, in consequence of which the 

 rhythmical pressure exerted in mastication and iu a constant direction 

 flattened and modified the primitive cusps of the teeth of the early Ter- 

 tiary bunodonts, causing them to become gradually selenodont, as we 

 actually observe to be the case when a series is examined the members 

 of which come from successively later and later horizons. 



The apparent correlation between the degree of complexity of the 

 crowns of the teeth of the equine series and the degree of digital re- 

 duction can be regarded as such only so far as two distinct mechanical 

 forces have oj^erated continuously and contemporaneously. Many other 

 instances of correlation may doubtless be similarly explained. 



When we look about us and find that certain phenomena are caused 

 by natural forces acting in certain well-defined uniform ways, we say 

 that the phenomena and effects produced are natural ones. That they 

 are determined in an approximately invariable manner we must infer 

 from the fact that the effects are alike or similar. Coincidences of this 

 kind occur in very great numbers in morphology, and there are few stu- 

 dents who have not had them forced upon their attention during their 

 investigations. Such series of similar effects imply the existence of 

 similar causes, as, for example, the presence of selenodont teeth in Un- 

 gulates which move the mandible laterally, and bunodont teeth in 

 those Ungulates which exhibit no lateral movements of the mandible. 

 Such coincidences are too numerous to be regarded as accidental, and 

 much as Dr. Tomes may desire to deny the real efficiency of a force gen- 

 erated by the muscles of an organism to produce changes of form in 

 already developed hard structures, such as enamel and dentine, such a 

 force nevertheless undeniably exists and produces its effects, as may be 

 inferred without chance of error from the fact of the very existence of 

 these very same numerous coincidences found in numerous recent and 

 fossil forms. Facts communicated to me by dentists show that abnor- 

 mal pressure may alter the form of a tooth during its growth and within 

 a comparatively short period ; yet Dr. Tomes has the hardihood to assert 

 with great assurance the very contrary, without ever once having prop- 

 erly read my papers or having analyzed the data involved in the dis- 

 cussion. 



Since natural selection cannot account for the origin of the reversal 

 of position of the crescents in the crowns of the opposing series of teeth, 

 for hundreds of other patterns would have answered just as well, or for 

 the invariable coexistence of such a crown-pattern with lateral laandib- 

 ular movements, in what manner are Dr. Tomes and those who agree 

 ^ith him to extricate themselves from the position in which their cal- 

 low haste has placed them I It is very clear to me that the awkward 

 position in which the English odontographer has placed himself will 

 sooner or later dawn upon his consciousness. 



The case of the teeth is a very strong one, and there can be no escape 

 from the conclusion that here, as in the case of the evolution of the 



