We may in mind and imagination walk in company with these teachers of ours; we may see the stages of the past of living creatures marked out in the geological strata, learning thus how life has grown from less to more, how its divergent streams have turned this way or that, and how some have been frustrate, as in the giant saurians and labyrinthodonts. We may learn too from the biologist that in our own day others (for instance perhaps those in the elephant and the ostrich and the whale) are in like manner drawing near to frustration by that impressive wearing out and perishing of a race, which is so clearly illustrated in the geological record. We discover that when impulsive life, seeking a free way, finds advance only through an excessive increase in the size of some creature, that creature's fate is sealed, and that manner of advance is barred. Size extending much beyond a certain range normal in the particular kind of animal is (perhaps in consequence of purely mechanical influences) to all the main intents and purposes of life no better than a blind alley. A like impulse, finding its outlet in the elaboration of nervous apparatus ranging from an all but indiscoverable structure to that definite and complex yet indefinitely mobile organ of an indefinitely extending choice—that living instrument for overcoming mechanism, that 'veritable reservoir of indetermination'—the human brain, finds there and there alone a road to great and full success. Yet it finds a road to minor and circumscribed successes in the lives of plants and animals which, interacting together, become of immeasurable value for the whole realm of life and the well-being of its citizens—for that realm of interlacing purposes and ends, where partial failure is so often converted to a new and a remoter triumph.