RECAPITULATION 201 



closely related to their functions in the new position 

 of the body. A mutation consisting in general 

 asymmetry would be comprehensible, but the head 

 of the Pleuronectid is not asymmetrical in a general 

 sense, but only so far as to allow of the changed 

 position of the eyes. The posterior end of the skull 

 is as symmetrical as in any other fish, and in some 

 cases the mouth and jaws are also symmetrical, 

 entirely unaffected by the change in the position of 

 the eyes. In other cases the jaws are asymmetrical 

 in a direction opposite to that of the eyes, there is no 

 change of position but a much greater development 

 of the lower half of the jaws, reduction, with absence 

 of teeth, of the upper half. In the latter case the 

 fish feeds on worms and molluscs living on the 

 ground and seized with the lower half of the jaws, 

 in the former the food consists of small fish swimming 

 above the Flat-fish and seized with the whole of the 

 jaws (Turbot, Halibut, etc.). 



I contend, then, that the mode in which the normal 

 Flat-fish develops is quite different from that in 

 which mutations arise. T. H. Morgan 1 states that 

 a variation arising in the germ-plasm, no matter 

 what its cause, may affect any stage in the develop- 

 ment of the next individuals that arise from it. In 

 certain cases this is true, that is to say, when there 

 are very distinct stages already. For example, a 

 green caterpillar becomes a white butterfly with 

 black spots. A mutation might affect the black 

 spots, an individual might be produced which had 

 two spots on each wing instead of one, and no sign of 

 this mutation would be evident in the caterpillar. 

 But my contention is that when this mutation 



1 A Critique of the Theory of Evolution (1916), p. 18. 



