SUCCESSION OF LIFE UPON THE GLOBE. 369 



and though there are apparent exceptions to the rule, there 

 are none of such a nature as not to admit of explanation. 

 Some of the leading facts upon which this generalisation is 

 founded will be enumerated immediately ; but it will be well, 

 in the first place, to consider briefly what we precisely mean 

 when we speak of " higher" and " lower" groups. 



It is well known that naturalists are in the habit of " clas- 

 sifying" the innumerable animals which now exist upon the 

 globe ; or, in other words, of systematically arranging them into 

 groups. The precise arrangement adopted by one naturalist 

 may differ in minor details from that adopted by another ; but 

 all are agreed as to the fundamental points of classification, 

 and all, therefore, agree in placing certain groups in a certain 

 sequence. What, then, is the principle upon which this 

 sequence is based ? Why, for example, are the Sponges placed 

 below the Corals; these below the Sea-urchins; and these, again, 

 below the Shell-fish ? Without entering into a discussion of 

 the principles of zoological classification, which would here be 

 out of place, it must be sufficient to say that the sequence in 

 question is based upon the relative type of organisation of the 

 groups of animals classified. The Corals are placed above the 

 Sponges upon the ground that, regarded as a whole, the flan 

 or type of structure of a Corai is more complex than that of a 

 Sponge. It is not in the slightest degree that the Sponge is in 

 any respect less highly organised or less perfect, as a Sponge, 

 than is the Coral as a Coral. Each is equally perfect in its 

 own way ; but the structural pattern of the Coral is the highest, 

 and therefore it occupies a higher place in the zoological scale. 

 It is upon this principle, then, that the primary subdivisions 

 of the animal kingdom (the so-called " sub-kingdoms ") are 

 arranged in a certain order. Coming, again, to the minor 

 subdivisions (classes, orders, &c.) of each sub-kingdom, we 

 find a different but entirely analogous principle employed as a 

 means of classification. The numerous animals belonging to 

 any given sub-kingdom are formed upon the same fundamental 

 plan of structure; but they nevertheless admit of being ar- 

 ranged in a regular series of groups. All the Shell-fish, for 

 example, are built upon a common plan, this plan representing 

 the ideal Mollusc; but there are at the same time various 

 groups of the Mollusca, and these groups admit of an arrange- 

 ment in a given sequence. The principle adopted in this case 

 is simply of the relative elaboration of the common type. The 

 Oyster is built upon the same ground-plan as the Cuttle-fish; but 

 this plan is carried out with much greater elaboration, and with 

 many more complexities, in the latter than in the former : and 



