VI DIFFICULTIES AND OBJECTIONS 129 



difficult cases, and this Mr. Darwin has done. One or two of 

 these may be briefly given here, but the whole series should 

 l)e carefully read l;)y any one who wishes to see how many 

 curioiis facts and observations have been required in order to 

 elucidate them ; whence we may conclude that further know- 

 ledge Avill probably throw light on any difficulties that still 

 remain.^ 



In the case of the mammary glands Mr. Darwin remarks 

 that it is admitted that the ancestral mammals were allied to 

 the marsupials. Now in the very earliest mammals, almost 

 before they really deserved that name, the young may have 

 lieen nourished hy a fluid secreted by the interior surface of 

 the marsupial sack, as is believed to be the case with the 

 fish (Hippocampus) whose eggs are hatched Avithin a some- 

 Avhat similar sack. This being the case, those individuals 

 which secreted a more nutritious fluid, and those whose 

 young were able to obtain and swallow a more constant supply 

 by suction, Avould be more likely to live and come to a healthy 

 maturity, and would therefore be preserved by natural selec- 

 tion. 



In another case which has been adduced as one of special 

 difficulty, a more complete explanation is given. Holes, 

 turbots, and other flatfish are, as is well known, unsym- 

 metrical. They live and move on their sides, the under side 

 being usually differently coloured from that which is kept 

 uppermost. Now the eyes of these fish are curiously distorted 

 in order that both eyes may be on the upper side, where alone 

 they would be of any use. It was objected by Mr. Mivart 

 that a sudden transformation of the eye from one side to the 

 other was inconceivable, Avhile, if the transit were gradual 

 the first step could be of no use, since this would not remove 

 the eye from the lower side. But, as Mr. Darwin shows by 

 reference to the researches of Malm and others, the young of 

 these fish are quite symmetrical, and during their growth 

 exhibit to us the whole process of change. This begins by 

 the fish (owing to the increasing depth of the body) being un- 

 able to maintain the vertical position, so that it falls on one side. 

 It then twists the lower eye as much as possible towards the 

 upper side ; and, the whole bony structure of the head being at 



^ See Origin of Species, pp. 176-198. 

 K 



