MAMMALS 



475 



of them are characterized. In both respects they contrast 

 markedly with Birds, and have a decided advantage over them. 

 For, given parents able to defend themselves, the sheltered 

 nature of viviparous development increases the chance of sur- 

 vival, and feeding the young on milk is a simpler matter than 

 collecting suitable nutriment for them, as so many birds have 

 to do, with expenditure of an enormous amount of energy. 



The three great groups of Mammals, beginning with the lowest, 

 are : Egg-laying Mammals (Prototheria), Pouched Mammals (Meta- 

 theria), and Higher Mammals (Eutheria). By successively re- 

 viewing these we shall obtain some idea of what has taken place 

 in the evolution of the class. 



EGG-LAYING MAMMALS (PROTOTHERIA). -The one family 

 (Monotremata) included in this group contains only the Duck- 

 Bill or Duck- Billed Platypus (Ornithorhynchus] and the Spiny 

 Ant- Eaters {Echidna and Proechidna] of the Australian region. 

 There can be no doubt that Australia was at one time con- 

 tinuous with the mainland mass of the Old World, but became 

 isolated before any of the higher Mammals migrated into it, owing 

 to which fortunate circumstance the lower members of the class 

 have been able to maintain themselves there, in the absence 

 of serious competition. Duck- Bills and Spiny Ant- Eaters alike 

 are of comparatively lowly structure, and possess many reptilian 

 characters, one of the most remarkable being that they lay 

 large tough-shelled eggs containing a great deal of food-yolk, 

 serving as a store of food for the developing embryo. 



The domestic economy of the best-known species of Spiny 

 Ant- Eater (Echidna aculeata) has of late years been the subject 

 of careful study, and is of unusual interest. The milk-glands 

 which distinguish Mammals from all other creatures almost 

 always have teats, from which circumstance, indeed, the name 

 of the class is derived (L. mamma, a teat). The Spiny Ant- 

 Eater does not possess these, although it has two milk-glands, 

 differing, however, in nature from those of other Mammals, the 

 allied Duck -Bill excepted. It commonly happens that when 

 " new " organs make their appearance they are in reality "old" 

 structures which have undertaken new work, and been corre- 

 spondingly modified (see vol. ii, p. 421). This appears to be 

 the case with milk-glands. We find in the skins of various 

 Mammals two important varieties of glandular structures, one 



