io 4 A BOOK OF WHALES 



first problem. There is, however, an initial difficulty 

 in the great superficial likeness which the various 

 members of the whale tribe bear to each other. It 

 needs no arguments to prove that the Mammalia 

 are essentially a land race other than those which 

 have already been advanced. To inhabit the water 

 is a mode of life entirely foreign to their organisation. 

 It is perhaps this which, in part at least, accounts for 

 the uniformity of structure which the large group of 

 whales exhibits. So little divergence from the suit- 

 able structure would be just the fatal straw. We find 

 as a support of this way of looking at the matter 

 similar uniformities in groups which inhabit an un- 

 usual medium. The group of birds, for example, 

 which contains an enormously large number of 

 different species, and is yet characterised by so great 

 a uniformity of organisation that the task of classify- 

 ing them has proved insuperable, is an example of a 

 race which has probably been modified to the aerial 

 life from a life among the branches of trees. Here 

 again a certain organisation is needful to live that life, 

 and wide departures from the most fitting type of 

 structure are not to be seen. 



A slight structural divergence might easily prove 

 fatal to the perfect fulfilment of their functions as 

 flying animals. Everyone is agreed that the orders 

 of birds are separated from each other by characters 

 of far less importance than those which separate 

 many, if not all, of the orders of the purely terrestrial 

 mammalia. The Cetacea, it is true, form but one 

 group equivalent to the Ungulata, the Rodentia, etc. 



