60 THE DARWINIAN THEORY 



undoubtedly at first sight a serious check, and is one 

 often referred to. By Darwin himself it was stated 

 to be the difficulty that would probably be most 

 widely felt. This difficulty must be considered 

 fully and from two main standpoints : First, 

 of what nature is the record yielded by fossils, 

 and how far is it reasonably complete ? Secondly, 

 are we quite certain that we know in what direc- 

 tion to look for intermediate forms, and are we 

 clear that we should recognise such if we found 

 them ? 



The geological record is imperfect for the follow- 

 ing reasons : — 



i. Only certain parts of certain animals can be 

 preserved as fossils. 



Among Protozoa the Foraminifera and Radio- 

 laria are well preserved, Infusoria not at all. In 

 Porifera the skeletons are well preserved, and of 

 these there is a long record. Of Cgelenterata 

 such forms as Hydra, jelly-fish, and sea-anemones 

 cannot be preserved, save very exceptionally ; but 

 corals are peculiarly suitable, and few classes are so 

 well represented in a fossil state. Echinodermata 

 are well represented, excepting the Holothurians. 

 Among Vermes there is no trace of flat or round 

 worms, but of Annelids the jaws, tubes, and tracks 

 are found. Arthropoda are well preserved, especially 

 the Crustacea, and as a general rule aquatic forms 

 are much more completely preserved than terrestrial 

 ones. In Mollusca the shell is well adapted for 

 preservation, but the air-breathing terrestrial forms 

 are rare. Among the Vertebrata we find the 



