138 Evolution and Adaptation 



of flatfish being bent towards the lower surface, with the jaw- 

 bones stronger and more effective on this, the eyeless side of 

 the head, than on the other side, for the sake, as Dr. Traquair 

 supposes, of feeding with ease on the ground. Disuse, on 

 the other hand, will account for the less developed condition 

 of the whole inferior half of the body, including the lateral 

 fins ; though Yarrell thinks that the reduced size of these fins 

 is advantageous to the fish, as ' there is so much less room 

 for their action, than with the larger fins above.' Perhaps 

 the lesser number of teeth in the proportion of four to seven 

 in the upper halves of the two jaws of the plaice, to twenty- 

 five to thirty in the lower halves, may likewise be accounted 

 for by disuse. From the colorless state of the ventral sur- 

 face of most fishes and of many other animals, we may 

 reasonably suppose that the absence of color in flatfish on 

 the side, whether it be the right or left, which is undermost, 

 is due to the exclusion of light." 



By falling back on the theory of inheritance of acquired 

 characters Darwin tacitly admits the incompetence of natural 

 selection to explain the evolution of the flatfish. If the latter 

 theory prove incorrect, it must then be admitted that the evo- 

 lution of the flatfishes cannot be accounted for by either of 

 the two main theories on which Darwin relies. 



Mivart further points out that the beginning stages of the 

 mammary glands cannot be explained by Darwin's theory. To 

 which Darwin replies, that an American naturalist, Mr. Lock- 

 wood, believes from what he has seen of the development of 

 the young of the pipe-fish (Hippocampus) that "they are nour- 

 ished by a secretion from the cutaneous glands of the sac" in 

 which the young are enclosed. This can scarcely be said to be 

 a satisfactory reply ; for, if it is true that this is the case for 

 the pipe-fish, — and I cannot find on inquiry that this state- 

 ment has been confirmed, — it is still rather speculative to 

 suppose that the ancestral mammals nourished their young by 

 secreting a fluid into the marsupial sac around the embryos. 



