and the Elements in which they live. 169 



aerial types in their perfect state of development, a large number 

 of which are aquatic but fluviatile in their larval condition, and 

 comparatively exceedingly few marine. So that if we compare 

 the whole type of Articulata with either the Mollusca or Radiata, 

 we see that in accordance with the higher development of its 

 structure it has not only proportionally a larger number of ter- 

 restrial and aerial types, but an entire class is throughout aerial 

 in its perfect state of development, and, though aquatic in the 

 stages of growth, the larvae are chiefly fluviatile and not marine ; 

 so that we may conclude from zoological evidence that the more 

 intimate connection with the main land and aerial mode of ex- 

 istence indicate a higher degree of development than an aquatic 

 mode of life ; and between the animals living in water, that fluvia- 

 tile types must rank higher than the marine. 



These views are fully sustained by the order of succession of 

 these great types of the animal kingdom throughout the earlier 

 geological periods ; for as it is already ascertained from zoological 

 comparisons, that the earlier types in each class rank lower than 

 their present living representatives, we have further evidence from 

 the circumstances under which they live that they were all aquatic 

 and marine in the earliest periods, and that fluviatile and terres- 

 trial tyjjes have followed only at later periods. Without alluding 

 to those classes in which the gradation of fossil types is less 

 distinctly shown, let me only recall the Crinoids among Echino- 

 derms, which for so long time prevailed to the almost entire ex- 

 clusion of all other families among Acephala ; the great preva- 

 lence of Brachiopoda in the oldest deposits and the first appear- 

 ance of Naiades in tertiary beds ; the large number of branchiate 

 Gasteropoda up to the time of the tertiary period, when Limncea 

 and Helices made their first appearance ; the earlier development 

 of Crustacea with more uniform joints, and the appearance of 

 insects of the tribe of Scorpions anterior to that of the winged 

 families, among which the Neuroptera seem to be the first to in- 

 crease in number, and the late occurrence of the sucking tribes 

 in tertiary beds, and there will be no doubt left that the grada- 

 tion of structure is intimately connected with the extension of 

 continental lands, and that the present^connection of animals with 

 the surrounding media in which they live agrees also with their 

 natural gradation. If we would study the natural relations be- 

 tween animals and the media in which they live, we could not 

 begin with better prospect of success than by investigating mi- 

 nutely the different families of Vertebrata separately, rather than 

 the whole classes of this great type. For though it is at once 

 apparent that the class of Fishes as a whole is entirely aquatic, and 

 stands at the same time lowest among Vertebrata, as soon as we 

 pass to the investigation of the Reptiles we find aquatic and even 



Ann. ^ Mag. N. Hist. Ser. 2. Vol. vi. 12 



