206 EDWARD MCCRADY, JR. 



After a similar surgical approach I have used a very small dental 

 drill to destroy parts of the cochlear duct (McCrady, Wever and 

 Bray, '37). After the lesion had been made in the inner ear a few 

 dermal sutures were made and the deeper structures were left to take 

 care of themselves. The sutures were sometimes covered with thin 

 celloidin. Aseptic precautions were observed throughout the opera- 

 tion, and healing was found to be very rapid. Survival was almost 

 100%. From 1 month to 3 months after the first operation a second 

 operation was performed for the purpose of making electrical tests 

 of the experimental ears. A comparison of the range of hearing 

 before and after such operations is in progress. 



The evolution of the mammals. One cannot examine a large 

 number of embryos of any species without pausing from time to 

 time to reflect upon the subject of evolution. The study of a par- 

 ticularly primitive mammal leads one almost irresistably to the sub- 

 ject of mammalian phylogeny. 



The theory which has been the basis for all discussions of this 

 subject in modern times was published by Prof. T. H. Huxley in 

 1880. He divided living and extinct mammals into three great sub- 

 classes the Prototheria, the Metatheria, and the Eutheria. Of these, 

 he conceived that the Prototheria in an unspecialized form evolved 

 first. They then split into two branches. One branch by a relatively 

 slight departure from the original stem became the modern Mono- 

 tremata. The other gave rise to the Metatheria, which in turn split. 

 One group of Metatherians by relatively slight specialization became 

 the modern Marsupials; the other gave rise to the Eutheria. So in 

 the modern world we have somewhat specialized representatives of all 

 three of the ancestral stocks the Monotremes are slightly specialized 

 Prototherians ; the Marsupials are slightly specialized Metatherians; 

 the Placentalia or Monodelphia are Eutherians in various degrees of 

 specialization. 



This has been, and is today, the most widely accepted theory of the 

 evolution of the mammals; but it has met with considerable opposi- 

 tion principally as a result of the discovery of many supposedly 

 Eutherian characters among the modern marsupials. Such discoveries 

 have led a few to believe that the modern marsupials are a degenerate 

 group descended from Eutherian ancestors (e.g., Dollo, 1898, and 

 Hubrecht, '08). 



Wilson and Hill (1897) and Hill (1897), while not accepting that 

 terminology, that is to say, not granting that the marsupials are 

 descended from Eutheria, at any rate, feel that they are descended 

 from a stock which was both diphyodont and placental. In other 

 words, they believe that the Metatheria developed a double dentition 

 and an allantoplacenta before they split up into the Marsupialia and 

 Eutheria. 



