DISTEIBUTION OF ORGANISMS. 185 



of organic being undergoing the modifi- s 



cations appropriate to its special consti- | 



tution, in connexion with the efficacy ^ " "^ 



upon that constitution of external con- ^ ^ 



ditions. m 



When we consider the Invertebrate 



animals in this arrangement, the ad- .2 - 



vances made in portions of the Animal g" 



Kingdom, before the end of the Silurian ^ 



era, appear in a very different light from g 



what they do while we entertain the ^3 



erroneous notion that there is but one -S 



chain of being. The annexed tabular 



view will make this readily and abund- g 



antly plain. Those animals which, 3 -| 



though hypothetically concerned in the g 



genealogy, are not actually found in a ^ 



fossil state, but for whose non-appear- 



r -i 8 



ance as iossils, reasons are given, are 3 .o 



expressed in italics. "%- -& 



In looking among the animals of one 

 class for the point of connexion by which 



it is joined to the next above, we must p 



" 



not invariably expect to find what we * % | 



are wanting in the highest species, for -3 



these are often the heads of branches. J H 



On the contrary, it appears, in many 



instances, in the lower species. And 'g . 



this is the more worthy of being pointed o 1 < J5_ 



out, as the supposition of something dif- -2 J 

 ferent has supplied one of the stumbling- ^ 

 blocks of the development theory. In 

 all the classes, for instance, which have 

 terrestrial as well as marine species, the 

 nexus to the next grade of being is among 

 the latter, which are invariably the infe- o 

 rior. And thus it is that no transition ^ 



of the kind here under our attention ^ < 



that is, none of the greater grade-transi- P3 < g 



tions takes place out of the aquatic ^ g | ^ 



medium which has here been regarded 9 ^ CO 



I .j I 



as analogous to that of all individual CO 5 



r*~) 



embryos. CO 



?; 



