Animals and the Eleme?its m which they Live. 207 



velopment of their structure, even some of the Vertebrata, 

 and, strange to say, we have among them only marine types, 

 not a single fluviatile representative, nor a single terres- 

 trial one. This fact would at first seem to be in direct 

 contradiction with the statements made before, if it were 

 not for the circumstance, that this class in itself, as repre- 

 sented in our days, does not seem altogether reduced in 

 comparison with the other two, if we could not be satisfied 

 that its perfect period of development were the former geo- 

 logical ages, when its numbers were far greater than at pre- 

 sent, a circumstance which places the whole class in peculiar 

 relations to its type, which must be rather appreciated under 

 the point of view of the conditions which prevailed in former 

 ages, when the ocean covered more extensively the whole 

 surface of the globe than at present ; so that the type with 

 its high organization must be considered more with reference 

 to its development in former ages, than to what it is now, as 

 at present, the class is proportionally reduced, and it is well 

 known, and it will be further mentioned with reference to 

 other types, that in earlier periods, however high animals 

 might have ranked by their structure, they were all marine, 

 as we know fishes to have been the only representatives of 

 Vertebrata in earlier periods. 



At this stage of the investigation, a comparison between 

 Mollusca and Radiata shews, that though the former advance 

 farther in their fluviatile development, and even reach with 

 some few of their types a terrestrial mode of existence, there 

 is not yet a single family among them which is entirely ter- 

 restrial, nor a single class which is either entirely fluviatile 

 or terrestrial, this connection with the higher conditions of 

 existence being only introduced among some few of their re- 

 presentatives, which we are allowed from other data to con- 

 sider as the highest in their respective groups. 



3. General View of Articulata. — If we now pass to the great 

 group of Articulata, and begin as before, by revising their 

 zoological arrangements, as based upon anatomical and em- 

 bryonic data, we shall have, at the outset, to settle the limits 

 of their classes, and their relative positions. 



The first point which we have here to investigate is the 



