56 DISCONTINUOUS VARIATION. 



teeth are dealt with in very great detail, and conclu- 

 sions are drawn as to the comparative frequency of 

 dental variation in various animals. The animals show- 

 ing the greatest frequency of extra teeth are domestic 

 dogs, Anthropoid apes, and the Phocidae. It is espe- 

 cially noticeable that the variability of domestic animals 

 in respect of teeth is not markedly in excess of that seen 

 in wild forms. Thus, though supernumerary teeth are 

 more common in domestic dogs and cats than in wild 

 Canidse and Felidse, they are not more so than in An- 

 thropoid apes and in the Phocidse. With respect to the 

 question of symmetry, the evidence shows that dental 

 variation may be symmetrical on the two sides, but that 

 much more frequently it is not so. Other evidence is 

 given concerning the division of teeth, the presence and 

 absence of first premolars and last molars, the least size 

 of particular teeth, and other subjects. 



Variations in the number of digits are treated more 

 fully than any other subject discussed, though the evi- 

 dence adduced is stated to bear rather on morphological 

 conceptions than in any direct manner on the problem 

 of Species. It is found that the frequency of digital 

 variation is immensely greater in some classes of species 

 than in others. Thus the horse shows many recorded 

 cases, but the ass none at all. Variation is common in 

 the cat, pig, fowl, and pheasant, but rare in the dog, 

 sheep, and in most birds. In the cat, ox, horse, pig, and 

 in man, the digital variation approaches to particular 

 forms, and has in it something distinctive. Digital 

 variation is sometimes symmetrical, but more often 

 asymmetrical. 



In other chapters of the book is found a considerable 



