634 



THE AMEBIC AN NATURALIST 



[Vol. LI 



not with an arbitrarily selected series of mutants is 

 further illustrated in the following note by Dr. Matthew. 



Of the hundreds of specimens examined no horse from the Lower 

 Eocene has ever been found which had any fully molariform premolars. 

 No horse out of the Middle Eocene has either more or less than one 

 molariform premolar in the lower jaw, on each side. Out of the Upper 

 Eocene all horses have two molariform premolars. In the Oligocene 

 all have three. All Oligocene and older horses have braehyodont molars 

 without cement. All Miocene horses are progressively hypsodont with 

 a progressive increase in the amount of cement. The milk teeth of 

 Miocene horses have almost no cement. Those of all Pliocene and later 

 hoi-ses are heavily cemented. At each successive stage of evolution the 

 cement appears at an earlier stage in the ontogeny of the tooth. These 

 are simply a few out of many progressive changes in the teeth, and 

 they are accompanied by equally clear progressive changes in the skull 

 and skeleton. Every one of these progressive stages is as exactly lim- 

 ited in time as the ones cited. 



Geneticists who are examining the nature of the paleon- 

 tological evidence regarding modes of evolution would do 

 well to realize that only a small part of the available ma- 

 terial bearing on the subject is either exhibited or pub- 

 lished. The scientific staff of the American Museum of 

 Natural History would be very glad to exhibit to their col- 

 leagues the great wealth of accurate data, concerning the 

 chronological sequence of specimens, which has been gath- 

 ered during twenty years of close exploration ; they would 

 also be pleased to place before them any of the extensive 

 series of specimens, sometimes amounting to several 

 thousands of individuals, which appear to throw light 

 upon the problem of continuity vs. discontinuity. 



In conclusion, paleontologists can show that evolution- 

 ary changes have involved progressive and measurable 

 emphases or suppressions of earlier structures or of ear- 

 lier proportions (allometric evolution, Osborn) ; and 

 when the progressive emphases are manifested as focal 

 outgrowths they seem like "new" structures (rectigrada- 

 tions, Osborn). Paleontologists, however, are not in a 

 position to say which characters would be transmitted 

 according to the Mendelian ratio, nor can they prove what 

 were the cytological causes of the evolutionary changes 



