Animals and the Elements in which they Live. 215 



elements in which animals live, the three other classes pre- 

 sent most diversified combinations, there being marine, fluvi- 

 atile, and terrestrial or aerial types in these classes, under 

 the development of as many structural types, differing almost 

 in the same degree when contrasted with each other, and so 

 much, that the aquatic Mammalia, even in their marine types, 

 or the marine turtles, differ as much from each other or 

 from bird, as they agree with their respective fresh-water 

 or terrestrial types. These discrepancies between the great 

 types may be owing to other motives in the plan of creation 

 than those to which they are here ascribed. The apparent 

 anomalies between some of the articulated types may also 

 be the results of combinations different from those with which 

 they are connected above. But whether these views are 

 correct or not, I have no doubt that the study of the pheno- 

 mena, which I am now contrasting, cannot fail to lead finally 

 to a more correct appreciation of the natural relations which 

 exist between animals and the media in which they live, than 

 the vague views which have prevailed lately from want of in- 

 vestigation of the subject rather than from an especial view 

 taken of it ; I am far from supposing that, in every instance, 

 I have hit at the outset the true view. I shall be satisfied to 

 have called forth direct investigation upon this question, and 

 led the way in a field which promises so ample reward. 



Magnitude of Animals. — Before entering into a special in- 

 vestigation of the natural relations of Vertebrata and the 

 surrounding media, it may not be out of place to call at- 

 tention to some collateral facts which will appear par- 

 ticularly prominent in the type of Vertebrata, but which 

 have already their value in the study of the lower types. 

 I allude to the relative bulk of animals of the same type 

 living in different media. We can derive no impression 

 upon this point from the investigation of Radiata, as they 

 are all aquatic, and almost entirely marine ; but the differ- 

 ence is already marked between Mollusca, if we contrast 

 their marine and their fluviatile and terrestrial types with- 

 in the limits of their natural secondary groups. Among 

 Acephala, if we consider the Lamellibranchiata, we cannot 

 fail to observe that the marine representatives are, as a 



