262 THE PHILOSOPHY OF BIOLOGY 



the remains of organisms are to be traced in the rocks, 

 but they have always been exclusively aquatic animals 

 and they are very sparsely distributed in fresh water 

 regions. Echinoderms are also a very old group, but 

 they were more abundant in the past than they are 

 now, and they appear to have been an exclusively 

 marine group of animals. Molluscs have existed since 

 the beginnings of stratified deposits and they are both 

 aquatic and terrestrial animals, but they belong pre- 

 dominantly to the sea. They have always been 

 relatively sluggish and inactive animals, with the ex- 

 ceptions of the great Squids and Cuttlefishes, but 

 fortunately for the other inhabitants of the sea these 

 formidable creatures appear to possess restricted 

 powers of reproduction, and they have never been very 

 abundant. All the smaller groups of animals are 

 restricted in their distribution : the flat-worms occur 

 sparingly both on the land and in the sea, and they 

 attain their highest development as parasites in the 

 bodies of other animals. Annelid worms, Gephyrea, 

 Nemertine worms, Polyzoa, Rotifers, etc., are all 

 groups of animals occurring mainly in fresh and sea 

 water and none of them is abundant. Related to most 

 of the great phyla are smaller groups : the extinct 

 Trilobites, Eurypterids, etc., in relation to the Arthro- 

 poda ; the group represented now by Peripatm in 

 relation to the Arthropods and Annelids ; the Entero- 

 pneusta and some other creatures which appear to 

 possess affinities with the Echinoderms and Chordates ; 

 and the extinct Ostracoderms, \vhich appear to have 

 been related to either the Arthropods or Vertebrates, or 

 to both. All these smaller groups of animals we must 

 regard as representing sidepaths taken by the evolu- 

 tionary process paths which have either ended blindly, 

 as in the case of those groups which have become extinct. 



