12 CLA&S MAMMALIA. 



and in investigating the cause of this extraordinary pheno- 

 menon, of this new order of things, lies the entire question 

 concerning the Marsupiata, it is necessary here to take the 

 arteries into consideration, which are agents of every or- 

 ganic production. We know that as well as the nutritious 

 vessels exist, so must also exist organs which they form and 

 supply. As there is but a certain quantity of arterial nutri- 

 ment to dispose of, if there be more than a due proportion 

 in one place, there will be less to distribute elsewhere. This 

 is that law of Nature which preserves a due balance between 

 the various parts of organization. 



In relation to the distribution of the arteries, there are 

 divers arrangements, some of which give to the Marsupiata 

 considerable affinities with the birds. The principal modifi- 

 cation is, that there is no lower mesentery in the abdominal 

 aorta. In the birds, this principal artery is carried behind 

 the iliacs. But in the marsupiata it is entirely wanting. 



The consequences of such a combination are, that from 

 the region of the reins to the rectum, there is no branch of 

 the abdominal aorta that may not, unless accidentally turned 

 from that purpose, concur in the business of generation. 

 In other mammalia, the lower mesentery, deriving in the 

 midst of the sources of life elements of another description 

 to return to the intestinal canal, is a cause at least of weak- 

 ening the generative operations. With the Marsupiata on 

 the contrary, (as with the birds where all the branches 

 springing from the abdominal aorta are similar, and em- 

 ployed without interruption to produce the same result, 

 having nothing to oppose their peculiar function,) they per- 

 form it of course with a proportional facility : and the energy 

 of their functions is not only increased, but each part yields 

 to a sort of reaction, the effect of which is to produce greater 

 activity, and more complete development. 



Another arrangement of very considerable influence is the 

 elevation of that point from which the abdominal aorta origi- 



