GREAT STEPS IN EVOLUTION 95 



It is not merely that plants in their struc- 

 tural relations remain about the level of 

 Ccelentera among animals; it is that they 

 are on an entirely different line of evolution. 

 Plants and animals are incommensurable and 

 antithetic. 



If we take a series of sedentary animals, 

 such as zoophytes or alcyonarian corals, we 

 find, as in plants, a wealth of variety within 

 narrow range, an exhausting of the possi- 

 bilities of ramification and colony-making, a 

 great development of hard supporting parts, 

 and many nice adjustments to slight environ- 

 mental peculiarities. They and the plants 

 have a similar kind of beauty expressing the 

 dream-smiles of their sleep-like life. 



How different this is from what we see 

 among the free-living animals which made 

 one important step after another. Keeping 

 to backboneless animals for the moment, let 

 us notice some of the great acquisitions 

 bilateral symmetry, a head-brain, specialized 

 sense-organs, a body-cavity, a segmented 

 body, muscular feet, a renewable external 

 armour, muscular jointed appendages, and so 

 on. Or let us think of particular cases such 

 as the extraordinary development of the res- 

 piratory system in insects, where ramifying 

 tubes carry air to every nook and cranny of 

 the body, so that the blood can hardly ever 



