These minute creatures are prolific to a degree that transcends all calculation; and they exist, either in the egg or maturely developed, in inconceivable numbers. A single wheel-animalcule, _Hydatina senta_, which was watched for more than eighteen days, and which lives still longer, is capable of a fourfold increase in twenty-four or thirty hours; a rate of propagation which would afford in ten days a million of beings. From their tenacity of life, extraordinary powers of reproduction, and incalculable numbers, their united influence may be said to be far more important, in all the great operations of nature, than that of the larger and more perfectly developed organisms. They swarm in all the seas, and play an important part in choking up harbours and forming great deposits at the mouths of rivers. The remains of those which have perished form great beds and strata in the crust of the earth. The silicious stone, called Tripoli, is entirely composed of such remains; at Bilin, in Bohemia, there is one stratum of this substance, fourteen feet thick, one cubic inch of which is estimated to contain forty-one thousand millions of individuals. Their extreme tenacity of life is evinced by the fact, that many of them may be entirely desiccated, and preserved in pure sand for several years, after which, on the application of a drop of water, they may be restored to life. In this dried state, M. Doyere exposed some of them to a heat equal to that of boiling water, and afterwards revived them; though, in an active state, if subjected to a much lower temperature, they perish. If, then, the fully developed and mature can resist such powerful extraneous causes of destruction, how much more must the ova possess the power of enduring them without losing their latent life! The following extract from Professor Owen's Lectures shows the bearing of these facts upon the question of equivocal generation.