XXI.] EMBRYOLOGY. 265 



the head. In some species these are magnificently deve- 

 loped, and, being filled with the creature's red blood, are 

 described as presenting the appearance of a carnation 

 flower. Yet the young of the tuhicolce are almost exactly 

 like those of the dorsihranchiata : their subsequent unlike- 

 ness is due to their afterwards fixing themselves, forming a 

 shell, and developing branchiaB on the head.' 



In a former chapter I have stated the fact of reversion, Keversion 

 that individuals are sometimes found which present what ^? ^ome- 

 appears to be the character of the ancestral form from 

 which all the species of a genu.s, or, it may be, all the 

 genera of a class, are descended. In this chapter we have 

 seen reasons for believing that the imaoe of that ancestral 

 form is in some degree preserved in the embryo or larva. 

 Consequently, if development is arrested, if the embryonic the reten- 

 characters are in some degree retained, while the organism Embryonic 

 in other respects becomes fit for mature Kfe, there will be characters. 

 a reversion to ancestral characters. In fewer words, re- 

 tention of embryonic characters is reversion to ancestral 

 characters. This is beyond doubt the explanation of some 

 cases of reversion, though I do not say that it will apply 

 to all. The best instance I know of is that of the flat- 

 fish or flounders, which differ from nearly all other verte- Flounders, 

 brates in their uusymmetrical form. They habitually 

 swim with one side u^jpermost, and both, their eyes are on 

 that side. But this is true of their mature forms only. Like 

 the Mollusca, they are symmetrically formed when in their 

 earliest state, and assume an unsymmetrical form in the 

 course of their subseqiient development. Young flounders 

 swim vertically, and have both sides alike, and one eye on 

 each side of the head, like fishes of the usual type. And 

 fully-grown individuals are sometimes found among the 

 various species of the tribe which continue to swim ver- 

 tically, and have their two sides less unequally developed 

 than is usual among flounders.^ Considered in reference to 



"O 



1 Carpenter's Comparative Physiolog}', p. 369. 



2 See Dr. Wyville Thomson on the Obliquity of Flounders, Annals of 

 Natural History, May 1865. He says of the wonderful change from the 

 young form, in which one eye is on each side, to the mature form, in which 



