TRI'lTDKRCULY IN I'll I. MATHS 59 



we come through the ages nearer to the present time \ve find that 

 the successors of those animals show a very much larger number 

 of cusps. How is this increase of cusps to be accounted for? II;is 

 there been a reserve store of conical teeth to increase the cluster? 

 No. Must obviously, to every student of the fossil history of cusps 

 there is no reserve store, but new cusps are constantly rising up on 

 the original crown itself by cusp addition. 



However, do not let me give you the impression that these 

 researches of Eose and Kukenthal are not of the greatest value 

 and interest : we shall see later on how the very facts of embryology 

 which are advanced by Dr. Carl IJose in support of his hypothesis 

 can be turned against him and used to support the differentiation 

 theory. 



I have no doubt many of you have observed, in the examination of 

 human lower molars, that occasionally instead of having four cusps 

 they have five. The fifth cusp always appears in the middle of the 

 heel, does it not, or between the posterior lingual and the posterior 

 buccal ? You find this in the monkeys and in many other mammals, 

 but I know of no record of the ancient anterior lingual reappearing. 



So we see that the human low r er molar tooth with its low, quadri- 

 tubercular crown has evolved by addition of cusps and by gradual 

 modelling from a high-crowned, simple-pointed tooth. Xow this, and 

 I say it with great confidence, is what has actually taken place. 

 It has not come about by bringing together single reptilian cones ; it 

 has been simply by the addition of one cusp after another to an 

 original single reptilian cone until there were six cusps, and then, 

 in the adaptation and fitting of the lower teeth to the upper, one 

 of the cusps has disappeared. This cusp was the primitive anterior 

 lingual, or, in comparative anatomy, the paraconid (Fig. 38, No. 8). 



Now let us follow the history of the upper teeth and see why 

 the " primitive anterior lingual," or paraconid, in the lower jaw has 

 disappeared. 



You are constantly in your practice, observing that one tooth in 

 the lower jaw gets into the way of another tooth and has to be pushed 

 out of place in order to place its opponent in the upper jaw into 

 its proper position. This is exactly what Nature has done ; Nature 

 has abandoned that lower cusp simply because, in the simultaneous 

 transformation of the upper teeth from a three-cusp to a four- cusp 

 type, there was no room for it. 



