322 THE ORIGIN OF SPECIES 



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 ex- 

 tremely remote period, and we can know nothing posi- 

 tively 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. It is admitted by most evolutionists that mam- 

 mals 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 (Hippo- 

 campus) the eggs are hatched, and the young are reared 

 for a time, within a sack of this nature; and an Ameri- 

 can 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 

 in the long run have reared a larger number of well- 

 nourished offspring than would the individuals which 

 secreted a poorer fluid; and thus the cutaneous glands, 

 which are the homologues of the mammary glands, would 

 have been improved or rendered more effective. It ac- 

 cords with the widely extended principle of specializa- 



