Chap. VII.] THEORY OF NATURAL SELECTION. 189 



the individuals possessing it, or favour their chance of having and 

 of rearing offspring." But there is no necessity for any such belief. 

 Habit, and this almost implies that some benefit great or small is 

 thus derived, would in all probability suffice for the work. 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 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 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 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 mam- 

 mals, and are indispensable for their existence ; they must, there- 

 fore, 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 cuta- 

 neous 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 mar- 

 supial sack. In the case of the fish (Hippocampus) the eggs are 

 hatched, and the young are reared for a time, within a sack of thia 

 nature ; and an American naturalist, Mr. Lockwood, believes from 

 what he has seen of the development 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 



