MR. OWEN ON THE GENERATION OF THE MARSUPIAL ANIMALS. 359 



But such explanations fail when the Jerboas of Africa are considered; for in 

 these animals we find similar proportions of the extremities, and consequently the 

 same kind of locomotion as in the Kangaroo, without any external pouch or 

 internal modification of the female apparatus indicative of a difference in their 

 generation from that of the ordinary Rodentia. And, on the other hand, the 

 ovo viviparous or marsupial il/«mma/m include the flying Petaurist, the burrowing 

 Wombat, the swimming Cheironectes, the climbing Koala, Opossums with the 

 hinder thumb and prehensile tail, and the Dasyures, with the ordinary proportions 

 and progression of the corresponding carnivorous genera of the placentally developed 

 Mammalia ; in all of which genera it is obviously impossible to connect marsupial 

 generation with the outward proportions, locomotion, or habits of the parent. 



Perhaps it is more philosophical to consider generation as having reference rather 

 to the whole nature of the thing generated, and its relative perfection as compared 

 with other species, than to partial modifications of the structure of the mother. 



The whole of the vertebrated animals are recognised as one great division or group 

 in nature, characterized by a plan of formation which, however varied to suit their 

 different spheres and powers of action, has sufficient basal or permanent characters 

 to be recognised as one type, distinguishable from that which pervades any other 

 lower organized group of the animal kingdom. 



But the generation most common to the vertebrated group is the same which 

 chiefly prevails in the lower divisions of the animal kingdom, viz. the oviparous, in 

 which the ovum, when once formed, detached, and impregnated, possesses properties 

 that enable it to accomplish all the steps of its future development, without further 

 connexion with the parent. The generation, therefore, which requires a second con- 

 nexion of the ovum to the parent, as in the placentally developed Mammalia, is an 

 exception to the rule of vertebratal reproduction, and we are led to inquire in what 

 essential points these animals deviate from or are superior to the other classes of the 

 division, that in their generation the parent should be subservient in a so much 

 greater degree to the perfect development of the new being. 



Now it is in the Mammalia that the brain is perfected : we can trace through 

 the different orders the increasing complication of this organ, until we find it in 

 man to have attained that condition which so eminently distinguishes him from 

 the rest of the class. And if the introduction of new powers into an organism ne- 

 cessarily requires a modification in its mode of development, with what other than 

 the perfection of the nervous system can we connect true viviparous or placental 

 generation ? for we do not perceive that in their digestion, circulation, respiration, 

 locomotion, or temperature, the Mammiferous Vertehrata are in any degree advanced 

 beyond the bird, in consequence of their more complex, or, as it may be termed, 

 more careful generation. 



Agreeably to this view, therefore, we should expect to find in those orders in 

 which the umbilical vesicle is largest and most permanent, and the placenta least 



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