366 PRLVCIPLES AND METHODS OF PALAEONTOLOGY. 



to be acquaintetl witli those principles of biological science wliich bear 

 most directly upon the subject: 



1. The most important of all the generalizations of natural history, 

 and, indeed, one of the most brilliant additions which the progress of 

 modern science has made to human knowledge, is the law that all ani- 

 mals and plants are associated and arranged according to certain fixed 

 laws. 



Thus, to select an example from the animal kingdom : There is an 

 immense variety of hoofed ruminating animals, antelopes, sheep, oxen, 

 deer, giraffes, camels ; but notwithstanding the extreme difference in 

 the aspect of these well-known creatures, the anatomist discovers that 

 they exhibit a great number of common characters. Thus — 



a. All jiossess a backbone, or vertebral column, separating the great 

 centers of the nervous system from those of the alimentary and circu- 

 latory apparatus, and the latter is situated on the ventral, front, or 

 downward face of the body; none have more than two pairs of limbs; 

 the chief central nervous system is not pierced by the alimentary canal. 



b. All have a heart with four cavities ; possess lungs and a midriff" or 

 diaphragm ; and have two facets on the hinder part of the skull, for ar- 

 ticulation Avith the foremost bone of the spinal column. In all, each 

 half of the lower jaw is in a single piece, and is articulated directly 

 with the skull by a convex head ; they all possess mammary glands for 

 suckling their young. 



c. The teeth are in all more or less deficient in the front part of the 

 upper jaw ; they all possess complex stomachs, and not more than two 

 completely developed long bones in the middle region of the fore and 

 hind feet. 



It would be easy to make a drawing embodying all these peculiari- 

 ties, and that drawing would stand in precisely the same relation to the 

 group of "ruminants" (technically called '-'■ BuminanticC) as the ground 

 I)lan of a single house does to the street which the architect means to 

 build of houses of that size and general form. The superstructure of 

 each house may, if the architect pleases, be totally different in style, 

 without in any way interfering with his general plan; and similarly, in 

 each particular ruminant, the common plan is preserved, while the de- 

 tails of the " elevation," the size, the figure, the proportions, the orna- 

 mentation in the way of color and horns, vary to an immense extent. 



Having thus acquired a notion of the "common plan" of the rumi- 

 nantia, it will be found, on turning to other equivalent groups or " orders " 

 of the MammaJla, (or animals which suckle their young,) that a corre- 

 sponding common jjlan may be found for each ; and when all these 

 common i^lans are compared together, it will be discovered that there are 

 certain respects in which they agTee. All mammalia, in fact, possess the 

 anatomical characters enumerated under the preceding heads a and b. 

 Hence, a drawing exhibiting these features would serve as a "common 

 plan" of the mammalia, and the common plans of the orders of mammals, 

 ruminantia,* carnivora, &c., might be regarded as modifications of the 

 plan of all mammals in the same sense as each ruminant is a modifica- 

 tion of the common i)lan of all ruminants. But now, if we were to extend 

 our researches further, and compare mammals with birds, reptiles, am- 

 phibia, and fishes, we should discover a still more remarkable fact, viz : 

 that all these creatures, and only these of all living tilings, possess the 

 character enumerated under the first head. Hence, a drawing or dia- 

 gram embodying these characters would represent the common plan of 



* Strictly speaking, the group Ruminantia is only apart of the modern order Artiodtic- 

 tyla, but it was convenient here to use the term in its old sense and vahie, or rather 

 sub-order Artiodactyla of the order Ungidata. ) 



