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- 



