2U INTRODUCTION. 



is remarkable for the development of its lateral lobes, and for the trans- 

 verse striae with which it is marked. In the brain of the Wombat (fig. 7) the 

 cerebral lobes (a, a,) are, in form and smoothness, like those of birds, and 

 do not cover the enormous olfactory lobes, or ganglions, which may be 

 seen projecting from the base of the cerebrum to some distance anteriorly 

 (d, d). In the Opossum the degradation of form is still more complete 

 the cerebrum is small, the olfactory lobes project greatly, and the optic 

 lobes, or corpora quadrigemina, separate considerably between the cere- 

 brum and the cerebellum. For additional valuable information on the 

 structure of the brain in marsupial animals, see Professor Owen, Phil. 

 Trans. Parti, for 1837. 



OSSEOUS SYSTEM OF MAMMALIA. 



WITH the perfection of the nervous system of the Mammalia is neces- 

 sarily connected a perfection in the totality of their organization ; but this 

 perfection, instead of arising from the multiplication of organs, is rather 

 the result of the progressive refinement of structures common to all the 

 vertebrata. It is by the modification of organs that Nature proceeds, 

 in her establishment of the grades of being ; and hence, whatever ex- 

 ternal or subordinate differences may be perceived among the higher 

 classes, a uniformity pervades the general plan upon which they are 

 fashioned. Nature, so to speak, appears to have proposed to herself 

 a model by which to be guided, and to the essentials of which she 

 has rigorously adhered. She has bound herself by laws which are 

 fixed and immutable : not that she has circumscribed her power ; not 

 that her forms are servile copies of some standard, from which she 

 never deviates ; on the contrary, her developments are as varied as her 

 resources she delights in a profusion of diversities : to the air, to the 

 earth, to the water, she accommodates the multitudes of her creation, 

 providing them, according to their necessities, with the means by which 

 they are enabled to fill their destined stations. All this is done with 

 an exquisite simplicity, a happy fitness, an ease and elegance, which 

 leave all human works and devices at an immeasurable distance. It is 

 effected, not by perpetual accessions of new parts, not by cumbrous ma- 

 chinery, to which, at every step, something has to be added, but by a 

 series of modifications, bearing immediate relationship to the great end 

 to be attained. 



It will be evident, upon consideration, that the general contour and 

 proportions of the body are determined by its internal osseous framework, 

 or skeleton ; for it is to this framework that the muscles are affixed ; 

 it is upon its parts, as levers, that they act ; and within it are enclosed 



