RECAPITULATION 219 



developed condition. It is very probable that in 

 the early stages of development the metabolism of 

 the body would be considerably different from that 

 of the adult stage, and the same combination of 

 hormones would not be present. We may suppose, 

 therefore, that the determinants of the zygote have 

 acquired a tendency to produce the increases and 

 decreases of tissue which constitute a certain modi- 

 fication, e.g. the change in the position of the eyes 

 in a Flat-fish, but the stimulus which caused this 

 tendency has always acted when the adult com- 

 bination of hormones was present. In consequence 

 of this the developed tissues do not undergo the 

 inherited modification until the adult combination is 

 again present. In this way we can form a definite 

 conception of the reason why an adaptive modifica- 

 tion is inherited at the same stage in which it was 

 produced, just as the antlers of a stag are only 

 developed when the hormone of the mature testis is 

 present. At the same time it is probable that the 

 age at which the inherited development takes place 

 tends to become earlier in later generations, to occur 

 in fact as soon as the necessary hormone medium is 

 present. 



The diagnostic characters of some of the species of 

 Pleuronectidae have been mentioned in an earlier 

 part of this volume, in order to point out that they 

 have no relation to differences of habit or external 

 conditions. Here it is to be pointed out that there 

 is no evidence that they arise by metamorphosis. 

 The scales, for example, afford distinct and constant 

 diagnostic characters both of species and genera, 

 but their peculiarities have not been found to arise 

 by modification of a primitive form. The rough 



