Brehm saw the young of an African monkey (Cercopithecus) clinging to the
under surface of their mother by their hands, and at the same time they
hooked their little tails round that of their mother. Professor Henslow
kept in confinement some harvest mice (Mus messorius) which do not possess
a structurally prehensive 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 climbing. I have received an analogous account from Dr.
Gunther, who has seen a mouse thus suspend itself. If the harvest 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 Cercopithecus, considering its habits while 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 manner 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.
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