214 L. Agas^iz on the Natural Relations between 



when Limnaea and Plelices 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 increase the number, and the late oc- 

 currence of the sucking tribes in tertiary beds, and there will 

 be no doubt left that the gradation 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 between 

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

 with better prospect of success than by investigating minutely 

 the diflferent families of Vertebrata separately, rather than 

 the whole class 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 marine types among turtles, which 

 rank much higher than the whole order of Batrachians, which 

 are almost entirely fluviatile ; and we find again marine and 

 fluviatile types among birds and Mammalia, the highest of 

 all Vertebrata. These facts shew, most conclusively, that an 

 organization as high as that of the Vertebrata, — introducing 

 a mode of existence so independent of the changes of the sea- 

 sons throughout the year, so durable as to last for numbers of 

 years (whilst among Invertebrata, and especially among in- 

 sects, but also among many other animals of lower type, there 

 exists the most intimate connection between their development 

 and the course of the seasons) ; we say these facts shew that, 

 with such animals which are placed so far above the influ- 

 ence of physical conditions, their connection with the cir- 

 cumstances under which they live is much weaker, so much 

 so, that internal structure overrules greatly the foundation 

 of those connections which are so intimate in lower animals, 

 and reduces their limits to subordinate connections between 

 members of the minor groups ; while, in the class of fishes 

 — the lowest — the whole type is organized in such a manner 

 as to make it uniformly dependent upon one of the natural 



