To ask of a new theory that it shall reveal to us exactly what took place in remote geological epochs, and how it took place, is unreasonable.

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.
In another case which has been adduced as one of special difficulty, a more complete explanation is given.