202 METAMORPHOSIS AND 



occurred, the original condition of one spot would 

 not be first developed and then gradually split into 

 two. Morgan proceeds to state clearly what I wish 

 to insist upon concerning mutations. He writes 

 that in recent times the idea that variations are dis- 

 continuous has become current. Actual experience, 

 he tells us, shows that new characters do not add 

 themselves to the line of existing characters, but if 

 they affect the adult characters, they change them 

 without as it were passing through and beyond them. 



Now in the case of the ancestors of the Flat-fish 

 the adult and the larva must have had the same 

 symmetry with regard to eyes and colour and the 

 dorsal fin terminated behind the level of the eyes. 

 Thus the variations which gave rise to the Flat-fish 

 were not discontinuous but continuous. In each 

 individual development now, not merely hypo- 

 thetically in the ancestor, the condition of the adult 

 arises by an absolutely continuous change of the eyes, 

 fins, and colour. Such a continuous change cannot 

 be explained by a discontinuous variation, i.e. a 

 mutation. The abnormalities above mentioned on 

 the other hand, although they doubtless arise from 

 the same kind of symmetrical larva as the normal 

 Flat-fish, and develop by a gradual and continuous 

 process, do not presumably pass through the con- 

 dition of the normal adult Flat-fish and then change 

 gradually into the condition we find in them. As 

 compared with the normal Flat-fish they arise by a 

 discontinuous variation, they are mutations, whereas 

 the normal Flat-fish as compared with its sym- 

 metrical ancestor arises by a continuous change. 



In order to make my meaning clear I must point 

 out that I have been using the word continuous in a 



