Animals and the Elements in which they Live. 213 



Among the sucking insects we begin again with various 

 aquatic types, or aquatic larval forms — next rise to Diptera, 

 with other aquatic larval conditions but a constant aerial 

 mode of life in the perfect state, and finally to the type Le- 

 pidoptera in which all larvae are terrestrial, and even highly 

 organized in their earliest state in the higher gi-oups, so that 

 the class as a whole does not only rank above Cinistacea for 

 its structure, but consists chiefly of aerial types in their per- 

 fect state of development, a large number of which are 

 aquatic, but fluviatile in their larval condition, and compara- 

 tively exceedingly few marine. So that if we compare the 

 whole type of Articulata, with either Moll u sea or Radiata, 

 we see that, in accordance with the higher development of 

 its structure, it has not only proportionally a larger number 

 of terrestrial and aerial types, but an entire class is through- 

 out aerial in its perfect state of development ; and, though 

 aquatic in the stages of growth, these larvae are chiefly 

 fluviatile and not marine, so that we may conclude from 

 zoological evidence that the more intimate connection with 

 the mainland and aerial mode of existence indicates a higher 

 degree of development than an aquatic mode of life ; and 

 between the animals living in water, that fluviatile 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 terrestrial types have fol- 

 lowed only at later periods. Without alluding to those 

 classes in which the gradation of fossil types is less distinctly 

 shewn, let me only recall the Crinoids among Echinoderms, 

 which for so long time prevailed to the almost entire exclu- 

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

 lence of Brachiopoda in the oldest deposits and the first 

 appearance of Naiades in tertiary beds ; the large number of 

 branchiate Gasteropoda up to the time of the tertiary period, 



