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 
be carefully read by any one who wishes to see how many 
curious facts and observations have been required in order to 
elucidate them; whence we may conclude that further know¬ 
ledge will probably throw light on any difficulties that still 
remain . 1 
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 
been nourished by 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 within a some¬ 
what 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, would 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. Soles, 
turbots, and other flatfish are, as is Avell 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, while, 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 
1 See Origin of Sgiecies, pp. 176-198. 
K 
