420 ROSS — ON EVOLUTION, 



development vi^ith gills, though at the present day only the lowest 

 Classes have any use for them, and many of the Amphibia only 

 before reaching the adult state. In succeeding Species of Ants 

 the wings may become embryonic and functionless. 



In the jRadiata, the planula {egg) of the Polyp, the Jelly-fish, 

 and the Star-fish, (representing the three chief divisions) are quite 

 similar, and in its early development a jelly-fish resembles a polyp, 

 while a Star-fish passes through stages in which it resembles first 

 the polyp and then the jelly-fish. 



The earliest (and lowest) known representatives of the Echino- 

 derms were the Crinoids, whose remains are found abundantly in 

 every formation from the Lower Silurian to the present day, when 

 they are represented by such Genera as Pentacrinus. All existing 

 Echinoderms pass through a Crinoid stage, and the higher and 

 later FamiHes pass through grades, representing the lower and older 

 successively until they attain their own proper grade at maturity, 

 when reproduction by the production of planulas occurs. In Penta- 

 crhius, again, the individual, after reaching its Crinoid stages, 

 passes through stages representing successive crinoid Genera lead- 

 ing up to itself. 



The mode in which most individuals of this Class are produced, 

 not by direct development from the Qgg, but by budding from other 

 individuals, so that a whole community has its origin in a single 

 Qgg, shews that in this as in some other respects the Radiata have 

 structural peculiarities akin to Plants. It is perhaps worthy of 

 remark that in Madrepores the top animal is always larger than the 

 side animals, whether in the stem or the branches, as are the buds 

 of a tree, the buds in either case beino^ most vitjrorous in the most 

 direct line, or in other words, less vigorous in direct proporiion 

 to the number of differentiations from the direct line of the 

 original polyp. 



At the base of the lower Silurian, the Cephalopods, Articulates, 

 and Radiates, disappear together, and save a few Fucoids a little 

 lower, no well ascertained organic remains have been found in 

 examining the rocks downwards through several miles in thickness. 

 It was for this reason that Hugh Miller perceiving that the lines of 

 organization (if I may use the expression) approximate as we trace 



