9 



assign to each genus its appropriate rank. I venture even to say, that 

 the time will come when the relative age of fossils, within certain 

 limits, will be as satisfactory a guide in assigning them their normal 

 position in a natural system, as the facts derived from the study of 

 their structure, so intimate are the connections existing between all 

 parts of the wonderful plan displayed in creation. 



Little or no advantage has as yet been derived from the study of 

 the relations of animals with the elements in which they live, in 

 ascertaining their natural relations among themselves ; but even in 

 this respect we may derive valuable hints from a careful study of 

 the geographical distribution of all animals ; and the mere nature of 

 the elements in which they live naturally. 



On reviewing lately the whole Animal Kingdom, with a view to 

 ascertain what is the value of the natural connection between the 

 animals and the media in which they live, with reference to organic 

 gradation, I have satisfied myself that aquatic types are decidedly 

 inferior to the terrestrial ; the marine inferior to the lacustrine and 

 fluviatile ones ; that those which live upon the main land and burrow 

 under ground are inferior to those which live above ground ; that 

 nocturnal types are inferior to diurnal types ; and that under otherwise 

 similar circumstances representatives of one and the same group, 

 which differ in these respects, have a higher and lower rank, in 

 accordance with their external circumstances ; so much *so, that 

 where we have no other guide, an influence respecting their natural 

 position may be fairly derived from their conditions of life. 



It will thus be obvious that as soon as we introduce simultaneously 

 into our classification considerations derived from all these different 

 sources ; as soon as we allow the embryonic development, geological 

 succession, geographical distribution, and relation to the natural 

 elements, to assist us in our efforts to assign to all animals a natural 

 position in one great system, we shall be able to sketch a far more 

 complete picture of the great diversity which exists in nature than 

 if we allow ourselves to be guided chiefly by anatomical data ; and 

 my object at present is mainly to urge the necessity of studies in 

 these different directions, with a view of improving our classification, 

 and to insist upon the necessity of keeping, at the same time, in 

 view, all these facts, whenever we attempt to form a correct idea of 

 the manifested relations which exist throughout the creation, from 

 the earliest period of the existence of animals up to the present day, 

 betw een all their different types. 



