240 A BOOK OF WHALES 



size of the teeth. It is, in the first place, hard to count 

 accurately the teeth in a given skull ; the smaller ones 

 at the ends of the series are sometimes lost, or 

 concealed in the gum. They become larger too with 

 growth, and more widely separated owing to the 

 growth of the beak already referred to. 



As both the numbers and size of the teeth are used 

 -and, in many cases, apparently quite properly used- 

 in the discrimination of species, it is desirable to be 

 cautious. 



In the third place sexual differences exist which, if 

 wrongly interpreted, might lead to the placing of the 

 two sexes in different species, when as has been more 

 than once the case a species is founded upon a single 

 individual, even upon a single skull or part of a skull. 



Fourthly, the distribution and depths of the colours 

 of these Cetaceans are apt to show differences, not 

 merely of age, but sheer variations which do not 

 always depend upon differing age. The Beluga, for 

 example, gets paler with age ; the arrangement of the 

 bars of colour upon the common dolphin, Delpkinus 

 delphis, seems to differ to a considerable extent. These 

 observations obviously apply to other whales besides 

 the Delphinidae, to which they are specially applied 

 here. Immaturity especially has been made the basis 

 of specific and even generic distinction. But they are 

 particularly applied to the dolphins by Sir W. Flower, 

 since the classification and limits of species in that 

 group are more difficult owing to their larger numbers. 

 In spite, however, of the numerous points in which 

 variation of sex or age may occur and tend to obscure 



