It is stated by some experimenters that ani malcules are produced when the infusions are exposed to hydrogen and nitrogen gases, or to atmospheric air artificially prepared ; in which it is held that there can be no living ova of animalcules. Again, it appears from numerous experiments, that when the infusions have been exposed to a boiling temperature, which is ge nerally believed to have the effect of destroying the life of all organized productions, the quan tity of animalcules formed is not diminished. Some air, it has already been stated, must always he present ; but so far as we are aware, the ex periments on this point have not been per formed in such a manner as to ascertain, whe ther or not, when an infusion is allowed to come in contact with a considerable portion of con fined air, and the whole apparatus is exposed to a temperature above that of boiling water, the production of Infusoria may still take place; and we are consequently obliged, in the absence of more direct experiment, to have recourse to analogical reasoning.
The following considerations appear to us to throw the balance of evidence in favour of the spontaneous production of Infusoria, mould, and the like.
Firstly, those organic matters which are most soluble in water, and at the same time most prone to decomposition, give rise to the greatest quantity of animalcules or cryptogamic plants.
Secondly, the nature of the animalcule or vegetable production bears a constant relation to the state of the infusion, so that, in similar circumstances, the same are always produced without this being influenced by the atmo sphere. There seems also to be a certain pro gressive advance in the productive powers of the infusion, for at the first the animalcules are only of the smallest kinds or Monocles, and after wards they become gradually larger and more complicated in their structure; after a time the production ceases, although the materials are by no means exhausted. When the quantity of water is very small and the organic matter abundant, the production is usually of a vege table nature; when there is much water,animal cules are more frequently produced.
Thirdly, on the supposition that infusory ani malcules are developed from ova, it is neces sary to conclude, from the experiments already referred to, that these ova are in some instances derived from the atmosphere, but yet the num ber of Infusoria is by no means in direct pro portion with the quantity of air. We arc also reduced to the necessity of holding that every portion of the atmospheric air is equally im pregnated with infusorial germs or ova, and that these bodies may remain for years dis solved, as it were, or invisibly suspended in the atmosphere, and in a perfectly dry state—a supposition contrary to analogy, and not fully warranted by the fact that NTibriones may be resuscitated by means of moisture after they have been kept in a dry state for long periods.
Fourthly, it may be remarked that the exist ence of ova, as belonging to many of the I nfu soria, is entirely hypothetical, since most of these animals are known, when once formed, to propagate by other means, as by the division of their whole bodies or by budding.
The production of infusorial animalcules from solutions of granite, silex, &c. recently described by Mr. Crosse, we have no hesitation in pronouncing to be either a mistake, or the result of changes occurring in admixed particles of organic matter.
The Entozoa, or that class of animals which live only in the bodies of others, afford proofs of spontaneous generation still more convincing than those already mentioned. These remark able animal productions are capable of existing no where but in the bodies of those animals which they naturally inhabit : they live either loose or attached, within cavities or imbedded in the substance of the textures ; sometimes in places, such as the alimentary canal or respira tory passages, to which the external air has access, and at other times in close cavities of the body, into which there is no opening from without, such as the chambers of the eye, the serous sacs, cysts, and other cavities, in the parenchyma of organs, the bloodvessels, &c. Entozoa do not live for any length of time after being discharged from the natural places of their abode ; and they survive a very short time only after the death of the animals in which they live.
If Entozoa are not admitted to be the pro duct of spontaneous generation, in order to ac count for their origin, it becomes necessary to suppose either that these creatures themselves or their ova pass directly from one animal to another, or that they are introduced through the medium of air or water. Upon the first sup position, carnivorous animals ought to be affected with entozoa, at least in greatest quan tity, if not in some instances exclusively ; and the entozoa infesting any particular animal ought to be of the same kind as those which exist in the animal serving it for food. But such is by no means the case. Herbivorous as well as carnivorous animals have entozoa, and in no less quantity, and each animal is the abode of its own peculiar kind. The same en tozoa infest the same animals in all localities and climates; thus all the human entozoa, with the exception of the Dracunctilus or Guinea worm, which is an external parasite rather than a true entozoon, are the same in all races of men. Neither do we recognise any simi larity between the entozoa infesting animals of a particular district and allied tribes of animals living in the neighbouring waters.