OVO- VIVIPAROUS MAMMALIA. 341 



on one hand, with the degree of intelligence manifested by the 

 animal ; and, on the other, with the manner in which the pro- 

 cess of reproduction is carried on in them. For in the brain of 

 Birds, whose early development receives little assistance from the 

 parent, the great central commissures (Amu. PHYSIOL., 434) 

 are altogether wanting ; whilst in the true Mammalia, in which 

 the embryo is sustained by a direct connection with the parent, 

 until it has nearly acquired the perfect form, the two hemi- 

 spheres of the brain are united by transverse bands, which 

 increase in number and complexity as we ascend towards Man ; 

 and in the Ovo- viviparous Mammalia, of which the embryo is 

 born prematurely (as it were), the brain attains a character but 

 very little higher than that of Birds, and the intelligence is 

 proportionably low. When it is recollected that intelligence, 

 depending on the high development of the cerebral mass, is the 

 characteristic feature of the class Mammalia, it becomes evident 

 that the group of animals, which differs from the rest in a pecu- 

 liarity so essential, should be completely separated from them. 

 And it is the less surprising that they should differ among them- 

 selves in characters which are elsewhere deemed important, since 

 we find the same occurring in other instances ; the lower or least 

 organised portion of a group often presenting sketches, as it were, 

 of the very dissimilar forms, which occur in the several divisions 

 above it. Thus in the Radiata, we may trace adumbrations or 

 shadowings-forth of the Molluscous, the Articulated, and even 

 the Vertebrated types ; and its several members differ far more 

 among each other in form and structure, than do the members of 

 either of the higher divisions of the Animal kingdom. 



307. By some of those who have recently attended to the 

 classification of this group, it is regarded as containing but a 

 single order, and is divided at once into families ; but in 

 general it is first subdivided into two orders, which will be here 

 adopted. The first of these contains the MARSUPIALIA or pouched 

 animals, so named on account of the peculiar pouch or pocket 

 which most of them possess at the lower part of the abdomen, 

 for receiving and protecting the young during the period of 

 suckling. This pouch is supported by two bones, termed the 



