TRANSLA TOR'S PREFA CE xiii 



leaning over to one side, without realising that the explana- 

 tion was Lamarckian. Now that it has been pointed out to 

 him that such an explanation is inconsistent with the theory 

 of natural selection, he admits that he can at present find no 

 explanation of the phenomenon which would be consistent 

 with that theory. 



What is the use of the scrotum in Mammalia ? In what 

 way is it better for the male mammal that its testes should 

 descend from their original position at the back of the 

 abdominal cavity into a membranous sac, where they are 

 constantly liable to injury ? vM*J ^jU^ <vrt* wJ n-^ 



What is the use of phosphorescence to pelagic animals ? 



What are the different conditions of life to which the 

 different structural characters of the classes of Echinoder- 

 mata are adapted ? The star-fishes have bodies consisting of 

 five, eleven, or thirteen, or some other number of radiating 

 arms connected by a central disc. In the Ophiuroidea the 

 arms are long, slender, and so brittle as to break at the 

 least resistance ; while in the Echinoidea the body has the 

 form of a hollow sphere. Are these differences adaptations ? 



It is generally assumed that the mechanism by which 

 flat fishes change their colour so as to assimilate it to that 

 of the ground on which they happen to lie is a beautiful 

 instance of adaptation. And yet the specific marking of 

 each species is perfectly distinct. A large number of 

 different species belonging to different genera live within the 

 same area on ground of uniform colour ; to this colour they 

 all have to assimilate their general tint, and yet each species 

 has its own permanent characteristic marking. As these 

 markings are all different, they cannot all aid equally in the 

 protective resemblance ; some of them at least must either 

 . 



