THE PROBLEMS OF MORPHOLOGY. 15 



severally taken advantage of by different types of creatures. 

 (1) To a given mass of moving matter the resistance of the 

 medium decreases along with decrease in the area of its 

 transverse section, and this implies increase of length : a given 

 force will move the lengthened mass along with greater 

 facility. (2) Reaching a certain point the elongated form 

 enables an animal to progress by undulations, as in the 

 water fish do, and even some ccelenterates and turbellarians 

 do, and as on land snakes do: lateral resistances serving in 

 either case as fulcra. (3) Lengthening of the body serves 

 otherwise to aid locomotion in the creeping or burrowing 

 worm, which, utilizing the statical resistance of its hinder 

 part thrusts onwards its fore part, and then, holding fast its 

 fore part by the aid of minute setce, draws the hinder part 

 after it. But elongation, doubly advantageous at first, while 

 the body is itself the chief instrument of locomotion, gradually 

 loses its advantageousness as special instruments of locomo- 

 tion are developed. (4) This we see in that locomotive 

 action effected by limbs, which, many and small in the lower 

 Arthropoda and becoming few and larger in the higher, at 

 length give great activity to a shortened and consolidated 

 body: a stage reached only through stages of decreasing 

 elongation accompanying increase of limb-power. (5) In 

 the Vertebrata locomotion by undulations comes, along cer- 

 tain lines of evolution, to be replaced by that limb-locomo- 

 tion which accompanies the rise from water-life to land-life : 

 the evolution of Amphibians exhibiting the transition. (6) 

 Further, we see among mammals that as limbs become effi- 

 cient the elongated body ceases to be itself instrumental in 

 locomotion, but that still some elongation remains a charac- 

 teristic. (7) Finally, where limb-locomotion reaches its high- 

 est degree, as in birds, elongation disappears. 



These classes of familiar facts I have recalled to show 

 that, in the course of evolution, achievement by plants of the 

 all-essential elevation into the air and by animals of the 

 all-essential power of movement have developed this trait 



