No. 3.] FACTORS IN THE EVOLUTION OF MAMMALIA. 399 



Further, the principle already quoted from Arbuthnot Lane 

 with regard to the skeleton, will doubtless apply to the teeth ; 

 " that a force which produces no obvious change in the skeleton 

 of the antecedents, but only a tendency to change, can, acting 

 as a developmental factor, cause the apparently spontaneous 

 development of that change in the offspring." The admission, 

 quoted by Professor Morgan, that "the new points do not 

 always occur where there has been previous abrasion," is due 

 to the fact that freshly formed and unworn teeth were examined 

 in that regard, overlooking later stages of wear. In all cases 

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

 stage, there is special abrasion. Of course, it would be folly 

 to pretend that the mechanics of dental evolution have been 

 completely worked out or that no difficulties exist. On the con- 

 trary, only a beginning has been made, but as a working JiypotJi- 

 esis, the assumption that the effects of use and disuse can be 

 inherited, and that mechanical factors do have a part in the 

 result, has already rendered such good service, that we feel 

 justified in retaining it. 



That the effects of use and disuse are not the only factors in 

 producing variations in tooth-structure, we have clear evidence. 

 Thus Nathusius has shown that in the swine defective nourish- 

 ment may have a remarkable effect in modifying the character 

 of the teeth. " Die Zahne des Versuchsthiers haben einen 

 ausserordentlich diinnen Schmelzuberzug, so diinn, dass derselbe 

 auf den Kauflachen von mol. 1 bereits vollstandig, von mol. 2 

 beinah ganz abgenutzt ist, wahrend bei dem gut genahrten 

 in Vergleich gezogenen Thier bei mol. 1 nur eine schwache 

 Abnutzung stattgefunden hat und bei mol. 2 sogar nur die 

 Haupthiigel des vorderen Jochs oben so weit ihre Spitzen 

 verloren haben, dass man in dem massiven Schmelzring einen 

 kleinen Knochenkern erkennt. 



"Dieser diinne Schmelzuberzug ist sehr wenig in Falten 

 gelegt, es ist dies besonders auffallend an dem noch in der 

 Hohle befindlichen mol. 3 dessen Haupthocker beinahe ganz 

 glatt sind. Alle Nebenhocker und Warzen sind schwach 

 entwickelt, an mol. 1 sogar kaum eine Spur davon vorhanden. 



" So hat denn dieser Versuch in frappanter Art ergeben, in 

 welchem Grade nicht nur die Starke des Schmelzuberzuges, 

 sondern auch das Zerfallen der Zahnkronen in zahlreiche und 



