58 EVOLUTION OF MAMMALIAN MOLAR TEETH 



if we reduce the number of its compound teeth, we find that the 

 most remote ancestors of these forms must have possessed more 

 than one hundred and fifty teeth. This number is certainly not 

 exaggerated, because Priodon, the giant tatusia (armadillo), a mammal 

 in an already quite advanced stage of evolution, possesses nearly one 

 hundred simple teeth, and in the dolphin this number rises from 

 one hundred and fifty to one hundred and seventy." I read this 

 to show that if there is any truth in the concrescence theory, Dr. 

 Ameghino partly deserves the credit for it. Moreover, we learn from 

 Schwalbe that the same theory was advanced by Professor Gaudry 

 in 1878, and still earlier by Professor Magitot in 1877. 



Now let me illustrate, in a very simple manner, what is meant 

 by the theory of concrescence and how we can imagine that the 

 human molars have been built up by bringing together a number 

 of isolated teeth. Placing a number of conical teeth in line, as 

 they lie in the jaw of the whale, they would represent the primitive 

 dentition. In the course of time a number of these teeth would become 

 clustered together in such a manner as to form the four cusps of 

 a human molar, each one of the whale-tooth points taking the place 

 of one of the cusps of the mammalian tooth, in other words, by a 

 concrescence, four teeth would be brought into one so as to constitute 

 the four cusps of the molar crown. Vertically succeeding teeth might 

 also be grouped. 



Now, what evidence is there in favour of this theory, and what 

 is there against it ? First, there is this, that all primitive types of 

 reptiles from which the mammalians have descended and many 

 existing mammals, as we have noted, have a large number of isolated 

 teeth of a conical form ; secondly, we find that by a shortening of 

 the jaw, the dental fold or embryonic fold, from which each of the 

 numerous tooth-caps is budded off' in the course of development, may 

 be supposed to have been brought together in such a manner that 

 cusps which were originally stretched out in a line would be brought 

 together so as to form groups of a variable number of cusps accord- 

 ing to the more or less complex pattern of the crown. 



What may be advanced against this theory ? This, and it is 

 conclusive to my mind : we find at the present time that cusps, 

 quite similar in ail respects to each of the cusps which form the 

 angles of the human molar, are even now being added to the teeth 

 in certain types of animals, such as the elephant, whose molar teeth 

 cusps are being complicated now or until very recent times. Then 

 we find in the Mesozoic period certain animals with tricuspid teeth. 

 Xow, according to the theory of concrescence these teeth ought not 

 to show any increase of cusps in later geological periods ; but as 



