RECAPITULATION 211 



metamorphosis is complete, namely, about one inch 

 in length, than the young Plaice or Flounder. It is 

 of little importance to consider whether at the 

 beginning of the evolution the change of position 

 occurred late or early in life. It may have become 

 earlier in the course of the evolution. The important 

 matter is to consider the evidence in support of the 

 conclusion that the relation to external conditions 

 has been the cause of the evolutionary change. We 

 have already seen that the nature of the change 

 and the relation of the change of structure to the 

 change of conditions necessarily tend to the infer- 

 ence that the latter is the cause of the former. 

 But we have to consider the particular changes in 

 detail. 



To take first the loss of pigmentation from the 

 lower side. I have shown experimentally that 

 exposure of the lower sides of Flounders to light 

 reflected upwards from below causes development of 

 pigment on the lower side. At the same time the 

 experiments proved that the loss of pigment in the 

 fish in the natural state and the development of 

 it under exposure to light were not merely direct 

 results of the presence or absence of light in the in- 

 dividual, for in some cases the young fish were placed 

 in the apparatus before the pigment had entirely 

 disappeared from the lower side, and the meta- 

 morphosis went on, the lower side becoming quite 

 white, and the pigment only developed gradually 

 after long exposure to the light. In the principal 

 experiment four specimens were placed in the 

 apparatus on September 17, 1890, when about six 

 months old and 7 to 9 cm. in length. One of 

 these died on July 1, 1891, and had no pigment on 



