3^4 GEOLOGICAL BIOLOGY. 



definite number of teeth of the reptilian jaw, is a natural 

 preliminary condition to the high specialization of the teeth, 

 with particular form for each. 



The selection and specialization seem to be brought about 

 by the suppression of part of the multiple series, and the 

 modification of the teeth retained in different parts of the 

 jaw for special function. 



In the primitive Marsupials and Insectivores, he observes, 

 the regular reptilian succession was early interrupted, while 

 in all the higher mammals the reptilian succession of two 

 series was retained in the anterior part of the jaw. In the 

 Edentates and whales retrogression takes place in fins as well 

 as in teeth; it is the first set of teeth that persists, the second 

 set being represented by a rudimental row of tooth-caps buried 

 in the jaw.* He concludes that there is strong evidence that 

 the stem mammals had a uniform number of each kind of 

 teeth and a uniform dental formula; that homodontism is 

 secondary, and was actually preceded in time by heterodont- 

 ism in the mammalian dentition. 



The ancestral formula for both Marsupials and Placentals, 

 according to this author, is: incisors 4, canines and pre- 

 molars 5, molars 4. By adopting Rose's suggestion that in- 

 cisor 5 of the marsupials belongs with the second series of 

 incisors, he supposes that Placentals have lost one incisor and 

 one molar from the primitive formula. The paper is an im- 

 portant contribution to the interpretation of the method of 

 evolution, and must be studied with care to be fully appre- 

 ciated; the author's conclusions are quoted on page 324. 



For the purposes of this treatise a sufficient number of il- 

 lustrative cases has now been presented to show where the 

 emphasis is placed by the facts of geological biology as to the 

 true factors of evolution. A great many examples crowd 

 themselves upon the attention which must be left for the 

 student to investigate directly and in detail. The evidence 

 to be derived from the study of living plants and animals is 

 so vast, that a special treatise would be necessary to do justice 



