COURTSHIP AND NURSERY DUTIES. 99 



variations which have gone on increasing in 

 intensity in the new direction, till it becomes 

 more and more marked ; and this of necessity, as 

 each new generation became further and further 

 removed from the old method of feeding. As a 

 final result, we get the highly specialised struc- 

 tures delicately adjusted to the purposes they are 

 required to fulfil. This adaptation to require- 

 ments we call specialisation. As instances of 

 specialised structures, we have the crushing teeth 

 of various kinds, the beak of the saw-fish and 

 sword-fish, and the remarkable tube-mouth of the 

 sea-horse, mormyrus, the curious tactile barbules 

 of the siluroids, and a hundred more. 



The importance of the part played by the 

 stimulus of hunger is shown by the fact that the 

 mouth parts of all animals vary most, and that 

 other modifications in the form of other parts of 

 the body are largely modifications connected with 

 the capture of the food. 



CHAPTER IX. 



COURTSHIP AND NURSERY DUTIES. 



THE period of courting or mate-hunting with 

 many fishes, as with birds, is signalised by 

 special activity on the part of the males. Some- 

 times this is manifested by quite unusual 

 aggressiveness ; sometimes by the display of 

 brilliant colours, combined very often with 

 greatly elongated, or otherwise specially modified 



