Seite - 244 - in The Origin of Species

244 ORIGIN OF SPECIES confinement some harvest mice (Mus messorius) which do not possess a structurally prehensile tail; but he frequently observed that they curled their tails round the branches of a bush placed in the cage, and thus aided themselves in climb- ing. I have received an analogous account from Dr. Giin- ther, who has seen a mouse thus suspend itself. If the har- vest mouse had been more strictly arboreal, it would perhaps have had its tail rendered structurally prehensile, as is the case with some members of the same order. Why Cercopi- thecus, considering its habits whilst young, has not become thus provided, it would be difficult to say. It is, however, possible that the long tail of this monkey may be of more service to it as a balancing organ in making its prodigious leaps, than as a prehensile organ. The mammary glands are common to the whole class of mammals, and are indispensable for their existence ; they must, therefore, have been developed at an extremely remote period, and we can know nothing positively about their man- ner of development. Mr. Mivart asks : "Is it conceivable that the young of any animal was ever saved from destruction by accidentally sucking a drop of scarcely nutritious fluid from an accidentally hypertrophied cutaneous gland of its mother? And even if one was so, what chance was there of the perpetuation of such a variation?" But the case is not here put fairly. It is admitted by most evolutionists that mammals are descended from a marsupial form; and if so, the mammary glands will have been at first developed within the marsupial sack. In the case of the fish (Hippicampus) the eggs are hatched, and the young are reared for a time, within a sack of this nature ; and an American naturalist, Mr. Lockwood, believes from what he has seen of the devel- opment of the young, that they are nourished by a secretion from the cutaneous glands of the sack. Now with the early progenitors of mammals, almost before they deserved to be thus designated, is it not at least possible that the young might have been similarly nourished? And in this case, the individuals which secreted a fluid, in some degree or manner the most nutritious, so as to partake of the nature of milk, would in the long run have reared a larger number of well-