In adopting the second supposition that the eggs or germs of Entozoa may gain the bodies of animals by circuitous routes, we are met by many difficulties in addition to those already stated in reference to a similar explanation of the origin of Infusoria. Many Entozoa reside only in particular organs of the body, and in the very interior of these organs, as the human Cysticercus cellulosus in the choroid plexus of the brain, in the substance of the brain itself, in the chambers of the eye, &c. so that it is necessary to suppose the ova of Entozoa to have been introduced into the circulation, carried through the smallest bloodvessels, and depo sited in the places in which they are developed. Animals living in the same situations and feed ing on the same substances have different kinds of Entozoa. The ova of some of the Entozoa, as for example, those of the common round worm, (Ascaris lumbricoides,) are so large that they could not pass through the largest even of the capillary bloodvessels the ova are so heavy that they could not be transmitted through the atmosphere ; and the supposition of the passage of the ova from parent to offspring is opposed by the mechanical difficulty of the transmission, as well as by the facts that parent and child are not always affected with the same kin& of worms, and that though the complaint of worms may be said to run in families, yet many escape, and one or more generations in the hereditary succession are frequently exempt from it. En tozoa have been observed in the fetus of ani mals, and supposing them to be introduced from without, it would be necessary to hold that the entozoa themselves or their ova have passed directly from the mother to the child in the uterus, or to have traversed a route through which the globules of the blood are not trans mitted.
Some of the Entozoa, we may further remark, when once formed, are viviparous or bear their young alive; and with regard to these kinds it would be necessary to suppose that they may arise by invisible ova or germs as well as pro pagate in the viviparous mode.
These facts appear to us to speak strongly in favour of the occasional occurrence of sponta neous generation,—" a doctrine which, had it not been applied in many instances where it was manifestly untrue, would have met with less ridicule and a more just appreciation than it has usually obtained." The epithet " spon taneous," which we have retained as the most common, is equally inappropriate as applied to this or to any other of the processes of' nature ; and the analogy of by far the greater number of plants and animals militates against the proba bility of the hypothesis ; but it must at the same time be held in mind that the organized bodies in which spontaneous production has been said to occur differ widely in their general structure and functions from those which are reproduced by means of ova; and we are scarcely entitled to reject the hypothesis of their spontaneous generation, merely on the ground that, in this respect, they do not agree with the rest of the animal kingdom. Harvey even, who established the proposition omne vivum ex ovo, seems yet to have acknowledged the ne cessity of admitting some difference between the more ordinary form of generation by means of an egg and that which he called of the spon taneous kind.* In conclusion, we may remark, that while we feel inclined to admit the existence of spon taneous generation among some species of cryp togainic plants, infusorial animalcules, and en tozoa, it must be held in recollection that many of these productions, after their first origin, propagate their species as parents,—that the so called spontaneous kind of generation is to he looked upon as no more than an exception to the general law of reproduction,—and that therefore extreme caution is necessary in admit ting any organized body to be the product of spontaneous generation upon the mere negative evidence of the absence of its seeds or ova.