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?
The extraordinary facts that have been discovered by Professors Steenstrup, Owen, and others, within a few years, respecting what they call _alternate generation_, or _parthenogenesis_, have been thought favorable to the hypothesis of development. Among the mollusca, the polyparia, the entozoa, and infusoria, it is found that, in some species, the result of sexual union is the production of a larva without sex, and, therefore, incapable of propagating in the usual way. Yet that larva can of itself produce another larva quite different from itself, and this larva another, and so on, sometimes for eight or ten generations, when the spermatic force seems to be exhausted, and a progeny exactly like the original parents that started the series is produced, capable of giving rise to another and a similar series. Here, then, we find a succession of progeny for several generations, and all quite unlike one another, yet without any immediate parental agency. Why is it not an example of spontaneous generation? and why may not new species be produced in this manner?