Chap. VII.] THEORY OP NATURAL SELECTION. 295 



rally 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 climbing. I 

 have received an analogous account from Dr. Giinther, 

 who has seen a mouse thus suspend itself. If the har- 

 vest mouse had been more strictly arboreal, it vrould 

 perhaps have had its tail rendered structurally prehen- 

 sile, as is the case with some members of the same order. 

 Why Cercopitheeus, 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 extreme- 

 ly 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 hyper- 

 trophied 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 mam- 

 mary glands will have been at first developed within the 

 marsupial sack. In the case of the fish (Hippocampus) 

 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 



development of the young, that, they are nourished by 

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