CKNKIIATION. 



431 



sary to conclude, from the experiments already 

 'I to, that thi'sr ova arc in some instances 

 derived (nun the atmosphere, but yet the num- 

 ber of Infusoria is by no means in direct pro- 

 portion with thu quantity of air. \Ve are also 

 reduced to the necessity of holding that every 

 portion of the atmospheric air is equally im- 

 ii-d with inl'iisnrial ucrms 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 

 .siipposiiion contrary to analogy, and not fully 

 warranted by the fact that VibrioMf may be 

 .tated 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 Infu- 

 soria, is entirely hypothetical, since most of 

 thc-c 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 

 desi i-ibed 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. 

 I .nio/oa do not live for any length of time after 

 being discharged from the natural places of 

 their abode ; and they survive a very snort time 

 onl\ after the deatli of the animals in which 

 they live. 



If Kntozoa 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 

 allcdrd with etitozoa, 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, > Miimal is the 



abode of its own peculiar kind. The same en- 

 to/oa infest the same: animals in all localities 

 and climates; thus all the human entozoa, with 

 the exception of the Dracunculus 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 animal-* of 

 i and allied tribes of animals 

 living in the neighbouring waters. 



In adopting the second supposition that the 

 eggs or germs of Kntozoa may gain the bodies 

 ol animals by circuitous routes, we are met by 

 litticullies 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 cellulosns in thu 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 thu 

 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 (acts that parent and child 

 are not always affected with the same kinds 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 fnctus 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 



