406 Causes and Course of Organic Evolution 



and as having been similarly derived in their past environal 

 history. 



The Reptilia are acknowledged by all zoologists to have 

 originated from fresh-water or terrestrial ancestors, for whether 

 we consider the flii)per-limbed plesiosaurians and ichthyosaur- 

 ians of the Lias, or the phases of paddle-limb formation seen in 

 the marine chelonians from the cretaceous period onward, 

 both are accepted as gradual modifications on a land ancestry. 

 The 5^28 genera therefore, and fully 5000 species fall under 

 that head. 



The 440 genera and at least 12,000 species of birds, as well 

 as the 494 genera and 5000 species of mammals, have equally 

 clearly originated as land forms. A synoptic table therefore 

 of genera that make up the Hemichordata, Urochordata, and 

 Chordata would show the following results: 



Group Fresh-water Marine 



genera genera 



Hemichordata 32 



Urochordata 130 



Cephalocordata 4 



Cyclostomata 1 5 



Gnathostomata 453 902 



Amphibia 171 



Reptilia 528 11 



Aves 440 



Mammalia 455 39 



Total 2048 1123 



^Miile mere statistics apart from historical e^ddence can 

 proverbially prove anything, we have further attempted briefly 

 to show the developmental trend that most have taken, where 

 this is fairly accurately knoT\Ti. The evidence indicates clearly 

 that some groups like the Spongida, the Enterocoela, the 

 Echinodermata, a considerable number of the Annelida and of 

 the Crustacea, a good majority of the Mollusca, all of the 

 Brachiopoda, and most of the Polyzoa amongst Invertebrata 

 are marine. But of these the morphological, the embryologi- 

 cal, and the geographical evidence, as well as in some cases 

 the palseontological, very strongly suggests that the Spongida, 

 the Enterocoela, the Annelida, and the Crustacea very early 

 originated as fresh-water, not as marine, organisms. 



