Animal Evolution. 199 



betokens an advance on what we may call the 

 " concertina " principle of locomotion, no great increase 

 of speed is attainable on this model either. Increase 

 of rings and legs only means more rings and legs to 

 move. A new departure was inevitable. Nature 

 must bestir herself and evolve something new. The 

 desiderata were concentration of motors, increase of 

 limb-length, better machinery for aerating the blood 

 and a central nervous bureau for instantaneous direc- 

 tion and control. In other words, nature must 

 construct a more complete electric and telegraphic 

 mechanism. 



These improvements were effected in two ways and 

 among two sets of jointed animals— one set aquatic 

 or pelagic, the crustaceans already mentioned ; the 

 other set terrestrial and aerial, spiders and insects. 

 They occur in a strange fashion in the caterpillar 

 itself. Thus after gorging for months and casting 

 skin after skin like the crabs, as it increases in fatness, 

 it at length on a final moult reappears as a mummy 

 or chrysalis. After hibernating in this natural 

 sarcophagus for months, resurrection ensues, and it 

 bursts into its final stage as a beautiful butterfly. 



Insects. — All insects metamorphose from the worm 

 or grub condition, hence we might assume that the 

 insect machinery is an evolution from the worm 

 mechanism ; but the two are quite distinct, the final 

 organic machine bearing little resemblance to the 

 sketch model. The insect body is divided into three 

 parts — the middle compartment being the engine 



