PRINCIPLES AND METHODS OF PALAEONTOLOGY. 367 



these auimals, which are collectively termed the Vcrtehrata ; and it would 

 stand in the same relation to the common plans of birds, mammals, rep- 

 tiles, amphibia, and fishes, as the ruminant plan did to oxen, sheep, and 

 antelopes. 



By carrying investigations of this kind into the rest of the animal 

 kingdom it has been shown that every animal whatsoever is a modifica- 

 tion of one or otherof five great common plans — the plan of the Vertebmta, 

 that of the Anmdosa, that of the Cwleiiterata, and that of the Protozoa. 

 This division of the animal kingdom is not generally adopted in this 

 country; that most prevalent recognizes the branches Vcrtehrata, Ar- 

 ticulata, Molnsca, Racliata, and Protozoa. 



It is most important, however, not to form a wrong idea as to the real 

 import of these "common plans." We must regard them simply- as de- 

 vices by which we render more clear and intelligible to our own minds 

 the great truth that the parts of living bodies are associated together 

 according to certain definite laws. Why it is that an animal which 

 suckles its young should invariably possess a double articular surface at 

 the back of its skull, should have the articular surface of its lower jaw 

 convex or flat and not concave, and should always be i)rovided with 

 hairs and never with feathers, we know as little as why the earth turns 

 from west to east, and not from east to west; but if the morphological 

 law which expresses this invariable coexistence, or correlation, of organic 

 peculiarities has been as regularly verified by our experience as the as- 

 tronomical law, we may, for all practical purposes, reckon as securely 

 upon the constancj- of oue relation as npon that of the other. 



It is, indeed, remarkable to how great an extent we may depend upon 

 these laws, and how seemingly unimportant, and in the present state of 

 physiology inexplicable, many of the most constant correlations of ani- 

 mal parts are. Thns the profoundest of "teleologists"* will, probably, 

 hesitate to attempt to account, by any physiological reasoning, for the 

 above-stated invariable occurrence of true hairs in those auimals only 

 which suckle their young and have two occipital condyles; but, never- 

 theless, if a single hair ha placed before a naturalist he will be able, in 

 many cases, not only at once to decide that the animal to Vvhich it belongs 

 possesses a backbone, has four limbs, suckles its young, has a heart with 

 four distinct cavities, possesses lungs ; but he may be able to go into miii ute 

 details as to the structure of its lirain, and the arrangement and number of 

 its teeth. How does he know these things ? Simply because experience 

 teaches him that the structure of the hair in question is found as a constitu- 

 ent part of only 07ie particular plan of organization, and, therefore, maybe 

 depended upon as an iudication of all the other peculiarities of that ]>lan. 

 Just as when a particular characteristic,' fossil is found we may predi- 

 cate what other fossils will be found in the same bed, without having 

 the least idea of the why and the v»herefore of the association; so thci 

 apparently trivial and unimportant hair indicates, we know not why, all 

 the other structural p-'culiarities which experience shows to be associated 

 with it. We shall find the application of these trutlis by and by in con- 

 sidering the methods by wliich fjssils are determiiicd. 



Important coasequeiices ilow from the fact that the forms of living 

 beings are modeled upon common })lans, and, from the kind of relation 

 wliich exists between any actual fortn and its j)lai!. Thus the vertebrate 

 plan, as has been seen, undergoes five modifications, each of which con- 

 stitutes the common plan of a large assemblage of animals — of mammals, 



* TF.LKOLOGy. — The doctrine of final causes. " Teleoloj^ist," one wLo ^eeks fur tlie final 

 causes of plienoineua. 



