226 GRAND STRATEGY OF EVOLUTION 



classes of animals which constitute this great trunk 

 line of organic evolution. 



The structural gaps which here and there divide 

 this long genetic series into classes, phylla, or orders, 

 generally represent relatively rapid organic improve- 

 ment in some particular which has quickly gained a new 

 stability and yielded large returns in minor variations 

 of it. 



For this reason we seek in vain to bridge them, es- 

 pecially the more creative ones, with the same degree 

 of completeness we do elsewhere, for these gaps are 

 probably real ones, accentuated by perspective, not nec- 

 essarily fortuitous gaps in our records, or in our knowl- 

 edge. 



These forward movements are rarely due to the 

 formation of new chemical substances, or to the acqui- 

 sition of essentially new powers by individual cells, for 

 the fundamental properties of life are very much alike 

 in all cell communities, high or low, large or small. 



Sense cells and nerve cells, for example, may not 

 be appreciably more delicate in their individual re- 

 sponses, or essentially different in their modes of ac- 

 tion, in the higher classes of animals than in the lower; 

 and there is apparently little difference in the actual 

 processes of digestion, in the making of new living ma- 

 terials, or in the production of bodily movements 

 through the agency of muscle, and nerve, and sense 

 organ. On the other hand, improvements in the social 

 life of cells, due to their cooperative organization, 

 may be very great indeed, and they afford the biologist 

 the most striking evidences of progress. 



