264 DARWINISM AND THE PROBLEMS OF LIFE 



The round worms do not represent the highest class 

 of the vermalians. There is a much more advanced 

 section, the annelides. To this group belongs the earth- 

 worm ; it has a very elaborate structure, but its marine 

 relatives are much more highly organised. They are 

 predatory animals with sharp eyes, and swim briskly in 

 the water in search of their prey. 



From the annelids have come the articulates : the 

 crustaceans on one side, and the tracheates on the 

 other. Of the former no transitional form has been 

 preserved, but this is not so with the latter ; in fact, 

 this particular animal, the peripatus, is the most typical 

 instance of a transitional form that we know. Half- 

 annelid and half-myriapod, it seems to have one organ 

 of the worm-type and another of the tracheate. The 

 peripatus is found in various species, but only a very 

 few ; and this is true of all transitional forms. It is 

 clear that animals of this kind, which have neither the 

 adaptations of their ancestors nor their descendants in 

 complete form, are easily crushed out by the two, and 

 can only be preserved in sheltered localities. When 

 we recollect, in addition, the eternal changing and re- 

 adapting in nature, we are surprised that any typical 

 transitional form has chanced to survive to our time ; 

 but we must not ask the theory of descent to justify 

 itself by producing actual instances of transitional forms. 



When we say that the vertebrates have been 

 developed from the vermalians, it must not be sup- 

 posed that any living animals, such as the maw-worms, 

 were their ancestors. We cannot assume that these 

 ancestors had relatives whose descendants are still 



