In a general survey of the life of this period, as it is revealed by the fossils, three outstanding facts are apparent: (I) the great divergence between the Cambrian fauna and that of the present day; (2) the Cambrian life assemblage differs in no marked manner from that of the succeeding Ordovician and Silurian periods; there is a certain family likeness which unites all of them; (3) the extraordinary complexity and diversity not only in the assemblage as a whole but within certain limited groups of organisms. Although in the Cambrian strata we have the oldest known fossiliferous rocks - if we leave out of account the very few and very obscure organic remains hitherto recorded from the pre-Cambrian - yet we appear to enter suddenly into the presence of a world richly peopled with a suite of organisms already far advanced in differentiation; the Cambrian fauna seems to be as far removed from what must have been the first forms of life, as the living forms of this remote period are distant from the creatures of to-day.
If the magnitude and increasing complexity of these creations fails to impress you, the sheer quantity should suffice.
The direct method of medusa-budding only differs from the polyp-bud by its greater complexity of parts and organs.
But in the Labyrinthodonta, grooves are more or less marked along the teeth and give rise to folds of the wall which, extending inwards and ramifying, produce the complicated structure, exhibited by transverse sections, whence these batrachians derive their name; a somewhat similar complexity of structure is known in some holoptychian (dendrodont) Crossopterygian fishes.
With this may be compared a passage in the Ursprung der Sprache, where there is a curious adumbration of Spencer's idea that intelligence, as distinguished from instinct, arises from a growing complexity of action, or, to use Herder's words, from the substitution of a more for a less contracted sphere.
They attained the highest degree of structural complexity in the Bennettiteae, which have been thought even to foreshadow a floral organization.
Knowing the almost endless complexity of organic structures, realizing that man himself with all the mystery of his life and consciousness must be included in any explanation of the origin of living things, they preferred to regard living things as something apart from the rest of nature, specially cared for, specially created by a Divine Being.
As an example of the complexity of this system we may note the five oxides of nitrogen, which were symbolized as the first three representing the gaseous oxides, and the last two the liquid oxides.
The gastrovascular system shows every degree of complexity from a very primitive to a highly elaborate type of structure.
In the dog it has been proved that after removal from the animal of every vestige of its cortex cerebri, it still executes habitual acts of great motor complexity requiring extraordinarily delicate adjustment of muscular contraction.