ORGANIC EVOLUTION 



239 



picture of evolution as we see it now. Some of the main 

 branches are clearly conjoined; others stand in doubtful 

 relationships. The ultimate origin of all of them is obscure, 

 for many of the older parts have perished. There is a 

 general divergence of the tips, but there is also convergence, 

 and even crossing. But there are enough long stretches of 

 unbroken growth to leave no doubt as to the general course 



FIG. 147. 



of progress, and there is enough convergence of all lines 

 backward to indicate that all the branches may have sprung 

 ultimately from a common source. 



Group radiation. Perhaps the most striking of the 

 phenomena of divergent development is that which has been 

 called adaptive radiation. This name serves to designate 

 that tendency seen in the members of all the larger groups 

 of organisms to become adapted to different natural func- 



