xl INTRODUCTION 



they could not have more than four. Similarly, insects 

 which do not fly have undergone a degeneration of the 

 wings. 



Birds, to continue Lamarck's examples, acquire webbed 

 feet by constant efforts to swim. In the course of these 

 efforts, the skin uniting the base of the digits is constantly 

 on the stretch. It therefore develops, and, in course of 

 many generations, birds are produced with complete webs. 



Other birds perch on trees, and by continual effort to 

 grasp the branches, develop long hooked claws. Others, 

 again, feed on water-animals or plants, but have a strong 

 objection to wetting their bodies. Hence they are constantly 

 stretching their legs, to keep their bodies above water when 

 standing in it. After many generations, long legs become 

 established, without feathers and adapted for standing in 

 water. For the same reason long necks were developed by 

 constant stretching. In the case of birds like swans, which 

 have no objection to swimming on the water, long necks 

 are estabhshed without long legs. 



Where the tongue is much used for reaching objects in 

 front, it also lengthens, as in the ant-eater or woodpecker : 

 where it has to grasp or palpate such objects, it becomes 

 cleft, as in humming-birds, lizards, and snakes. 



Flat fishes normally swim in the vertical plane, with one 

 eye on each side of their heads. In certain species, however, 

 the fish desires to come as near as possible to the shore ; 

 and in the case of a shelving beach, it can approach much 

 nearer land by turning on to its side and swimming in the 

 horizontal plane. One eye, being then on the under-surface, 

 where there is really nothing worth looking at, is perpetually 

 being strained or twisted, round to see what is going on 

 overhead. Hence, after many generations, it moves bodily 

 round to the upper side, as in soles, turbots and dabs. 



Herbivores are commonly massive because they spend 

 all their time eating : some, however, such as gazelles, are 

 liable to be pursued by carnivores, and hence acquire legs 

 capable of rapid locomotion ; others, such as deer, develop 



