Not many years since, the equivocal or casual production of animalcula, without any other parentage than law, was thought to be made out by a multitude of facts. For these minute creatures appeared almost every where, and in places where it seemed impossible that their ova should be found. But the researches of Ehrenberg have cleared up the difficulties of their origination in the ordinary modes of reproduction, in nearly every instance, and the advocates of the law hypothesis have been fairly driven from this stronghold of their argument. In describing the various modes of reproduction with which nature has provided the infusoria, Professor Owen says, "Thus each leaves, by the last act of its life, the means of perpetuating and diffusing its species by thousands of fertile germs. When once the thickly tenanted pool is dried up, and its bottom converted into a layer of dust, these inconceivably minute and light ova will be raised with the dust by the first puff of wind, diffused through the atmosphere, and may there remain long suspended; forming, perhaps, their share of the particles which we see flickering in the sunbeam, ready to fall into any collection of water, beaten down by every summer shower into the streams or pools which receive or may be formed by such showers, and, by virtue of their tenacity of life, ready to develop themselves whenever they may find the requisite conditions of their existence. The possibility, or, rather, the high probability, that such is the design of the oviparous generation of the infusoria, and such the common mode of the diffusion of their ova, renders the hypothesis of equivocal generation, which has been so frequently invoked to explain their origin in new-formed natural or artificial infusions, quite gratuitous."--_Lectures on Comp. Anat._ vol. ii. p. 31. No longer able to maintain a foothold among the animalcula, the defenders of this hypothesis have of late attempted to take a stand among animals of a somewhat higher grade, viz., the entozoa, or animals inhabiting other animals. These being considerably larger than the infusoria, their ova could not float in the atmosphere; but they possess a wonderful tenacity of life; some of them exhibiting signs of life after having been in boiling water for an hour; others have revived after having been packed for a long time in ice, and frozen; others have revived after lying in a dried state for six or seven years. Their power of reproduction, in the ordinary modes, is also prodigious, exceeding even that of the infusoria. It will, then, demand very strong evidence to prove that such animals possess also the power of spontaneous production, without parentage, or that their existence within other animals cannot be explained without such a supposition. For, if capable of being produced without parentage, why should such extraordinary care have been taken for their multiplication, in almost all the ordinary modes in which animals are reproduced?