in EXTREME SPECIALISATION 183 



that a fully -formed land animal, with 

 limbs completed for walking on the land, 

 has given birth to offspring which aban- 

 doned the use of them, and acquired, by 

 nothing but ordinary generation, all the 

 purely marine adaptations of the Whale. 

 There is, perhaps, no creature so 

 highly specialised. The baleen in the 

 mouth is one of the most wonderful cases 

 of an organic apparatus expressly made 

 for one definite and very peculiar work 

 namely, that of forming a net or sieve 

 for entangling and catching the millions 

 of minute crustaceans and other organ- 

 isms which swarm in the Arctic seas. It 

 is one of the structures which classifiers 

 call aberrant cases in which the direc- 

 tive agency so evidently supreme in all 

 organic development has pursued a 

 certain line of adaptation into the rarest 

 and most extreme conditions determined 

 by a very peculiar food. In the pursuit 



