THEIK POSITION IN THE ANIMAL KINGDOM. 3 



The position of the Mammalia in their connec- 

 tion with the other classes of vertebrates, and their 

 special relationship with one class of them taking 

 the word relationship without its explanation by 

 descent rendered the old system extremely obscure, 

 for the facts themselves spoke very imperfectly. 

 The anatomical resemblance is nowhere so absolutely 

 distinct that the descriptive method could point to 

 the bird, or to any one of the vertebrates, as show- 

 ing the closest and most numerous affinities with 

 the Mammalia. That animals of the frog species 

 and mammals should possess two condyles at the 

 back of the head as the first vertebrae of the neck, 

 and that birds, like lizards and their relatives, had 

 only one, always seemed surprising to comparative 

 anatomists. Yet what was to be done with this 

 and other similar facts ? We know that since the 

 middle of the eighteenth century the idea of descent 

 and development flickered up here and there. But 

 the theory was unable to rise above the general 

 conception, and even in the present century its 

 importance was not felt by an anatomist who, in 

 my opinion, nevertheless possessed the clearest 

 ideas of the connection between the present and the 

 past. I refer to the elder d' Alt on. Although his 

 interest in the subject was aroused by Goethe, it 



