RECAPITULATION 205 



It is perfectly true, as Morgan says, that new 

 characters which arise as discontinuous variations 

 — in other words, those kinds of variation which are 

 called mutations — do not add themselves to the line 

 of already existing characters, but ' change the 

 adult characters without as it were passing through 

 and beyond them.' The mutations which Morgan 

 describes in his own experiments on Drosophila 

 illustrate this in every case. In no case is the 

 original organ or character, e,g, wings, of the normal 

 Fly first developed and then changed by a gradual 

 continuous process into the new character. It 

 might perhaps be said that this took place in the 

 pupa, but that seems impossible, for the complete 

 wing is not fully developed in the pupa. The same 

 truth is equally apparent in the mutations described 

 in (Enothera. It follows, therefore, that none of 

 the evolutionary changes which have produced what 

 are called recapitulations can have been due to 

 changes of that kind which is known as mutation. 



The abnormalities in Pleuronectidae to which I 

 have referred are of the kind usually regarded as 

 due to arrested development. But closer considera- 

 tion gives rise to doubt concerning the validity of 

 this explanation. It might be supposed that the 

 attached base of the dorsal fin is unable to extend 

 forward because the eye on the edge of the head is in 

 the way, but if the metamorphosis is arrested, why 

 should the fin grow forward in a free projection ? I 

 have described a very abnormal specimen of Turbot 

 in a paper communicated to the Zoological Society of 

 London,^ and in that paper have discussed other 

 possible explanations of these mutations. In the 



» Proc. Zool. Soc, 1907. 



