The most that should be asked is, that some
probable or possible mode of origination should be pointed out in some
at least of these 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 knowledge will probably throw light on any
difficulties that still remain.[41]
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 somewhat 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
selection.
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