GENERAL CHARACTERS OF MAMMALIA. 63 



43. This high development of the intelligence in Mammalia, is evidently 

 connected with the greatly-prolonged connection between the parent and the 

 offspring, which we find to be a characteristic of this class. Mammalia are, 

 like Birds, warm-blooded Vertebrata, possessing a complete double circula- 

 tion ; and some of them are adapted to lead the life of Birds, passing a large 

 part of their time in darting through the air on wings, in pursuit of Insect 

 prey. But they differ from Birds in this essential particular, that they are 

 not oviparous, but viviparous; producing their young alive, that is, in a 

 condition in which they can perform spontaneous movements, and can appro- 

 priate nourishment supplied to them from without. But they are not distin- 

 guished from all other animals by this character alone ; for there are some 

 species among Reptiles, Fishes, and even Insects, which produce their young 

 alive, the egg being retained within the oviduct and hatched there. The 

 real distinction lies partly in that, which the name of the class imports, the 

 subsequent nourishment of the young by suckling ; and partly in the mode 

 in which the embryo is nourished before its birth. In Mammels, the yolk- 

 bag is very small in proportion to its size in Birds ; and the contents of the 

 ovum, instead of furnishing (as in that class) the materials necessary for the 

 development of the young animal, up to the time when it can ingest food for 

 itself, only serve for the earliest set of changes in which this process con- 

 sists. In the latter stages of the evolution of the embryo, it is supplied with 

 nutriment directly imbibed from its parent. This is at first accomplished 

 by means of a series of root-like tufts, which are prolonged from the surface 

 of the ovum, and insinuate themselves among the maternal vessels, without, 

 however, uniting with them. These tufts absorb, from the maternal fluid, 

 the ingredients necessary for the support of the embryo ; and also convey 

 back to the parent its effete particles, which are received back into her blood, 

 and are then cast out of her system, by the process of secretion, respiration, 

 &c. 



44. The Mammalia may be divided into two sub-classes ; in one of which 

 the structure just described is the greatest advance ever made, in the appa- 

 ratus by which the fcetus is nourished ; whilst in the other a more concen- 

 trated form is subsequently assumed by it. The ovum of the latter is delayed 

 for a longer period, in a cavity formed by the union of the two oviducts, 

 termed the uterus; which can be scarcely said to be developed in the Marsu- 

 pialia and Monotremata, the two orders constituting the first sub-class. The 

 vascular tufts proceeding from the chorion become especially developed at one 

 point, and the vessels of the uterus are extremely enlarged in a corresponding 

 situation ; the tufts dip down, as it were, into a chamber formed by an exten- 

 sion of the inner lining of these vessels, and serve the combined purpose of 

 the roots of plants and of the branchiae of aquatic animals, absorbing from 

 the maternal blood the materials required for the nourishment of the embryo, 

 and aerating the blood of the fcetus, by exposing it to the influence of that of 

 the parent. The peculiar organ thus formed is termed the placenta ; and the 

 two sub-classes of the Mammalia have thence received the appellations of 

 placental and non-placental. The animals belonging to the latter present 

 many points of affinity to Birds, in the structure of their internal organs. 

 That of the brain is very nearly allied in these two groups ; and their amount 

 of intelligence seems, as far as can be determined, to bear a close correspond- 

 ence. The Ornithorhyncus in particular, has so many marks of alliance to 

 oviparous animals, and its osteology, as well as in its horny bill and in less 

 important particulars, that Naturalists have much debated whether it could 

 really be termed a Mammiferous animal. No positive evidence has yet been 

 obtained that its young are born alive ; but on the other hand, there is a strong 

 reason to believe that they come into the world uninclosed in the ovum, al- 



