124 OTHER FACTORS OF EVOLUTION 



bottom of the sea, though they are well known to have a perfect 

 bilateral symmetry when young, like that of ordinary fishes. 

 Cunningham also attaches much importance to these changes. 

 Thus he says : — "The theory of independent variation and selection 

 as applied to flat-fishes is unsupported by evidence, while the con- 

 clusion that the metamorphosis of these fishes is the direct result 

 of the change of conditions is in harmony with all that we know 

 of the effect of physical conditions on individual organisms." ' He 

 cites also another remarkable case in which particular habits seem 

 to have engendered a definitely corresponding change in another 

 fish (Anableps) living in the estuaries of Brazil and Guiana, which 

 " does not wear spectacles, but actually has its eyes made in two 

 parts, the upper half of the lens having a different curvature from 

 that of the lower. The pupil is also divided into two by pro- 

 longations from the iris." And the explanation of this remarkable 

 condition seems to lie in the fact that, " this fish is in the habit of 

 swimming at the surface with its eyes half out of the water, and 

 the upper half of the eye is adapted for vision in the air ; the lower 

 half for vision under water." There is no reason, he says, to 

 suppose "that the required variations ever occurred until the 

 ancestors of Anableps took to swimming with their eyes half out 

 of water" (loc. cit. p. i6). 



Haeckel likewise maintains that the history of parasites 

 "provides an abundance of the most striking proofs of the 

 much-contested inheritance of acquired characters." ^ No other 

 circumstance has so profound an influence on the organism as 

 adaptation to a parasitic existence. Moreover, as he adds, " there 

 is no other section in which we can follow step by step the course 

 of the degeneration which is caused, and show clearly the 

 mechanical nature of the process." This may be fully admitted 

 without denying the share that Natural Selection often takes in the 

 process. 



These cases ought rather to be refeiTcd to under the next 

 head of changes wrought by alteration in the conditions of life. As 

 examples occurring in animals, however, it is more convenient to 

 refer to them here, and also to two other cases showing the 

 remarkable changes that may be brought about in some of the 

 embryos of bees and ants by mei^e variations in the food with 

 which they are suppHed. There is first the well-attested fact of 



■ Loc. cit. p. 23. 



= " The Wonders of Life," p. 243. 



