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 flax 
or type of structure of a Coral 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 A7ollusca, 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 
2A 
