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the course of waters, which has been noticed in the spread of the disease; one of the few facts regarding it which seem to

have acquired any thing like certainty, though not free from the contradictions and anomalies which press on every part of the subject. Again, the frequent occurrence of insulated cases of Cholera, before the wider breaking out of the disorder in any given locality, though occasionally it may be attributed to infection, yet has peculiarities analogous to the habits of the swarms of aphides, and other insects, infesting plants; and there are many instances for which it would be difficult to find any more plausible solution.

The reappearance of the disease in the same locality, at uncertain intervals, but generally during the hotter part of the year, is another fact bearing on the same hypothesis. The cases have usually been more detached than on the first ingress of the malady, and much less numerous, but often as fatal. Difficult of solution on any hypothesis, these facts are less so perhaps on the supposition of an animal virus than on any other. It offers the contingencies of fresh swarms arriving; or of the developement of ova deposited in these places during the preceding incursions of the disease, and called more or less numerously into life by increased temperature or other causes. + The latter view accords better

*Remarkable exceptions occur to this, as at Berlin; where the epidemic of 1837, notwithstanding it was six weeks shorter in duration than that of 1831, attacked a proportion of persons greater by one third, and with a somewhat higher rate of mortality; showing that the virus was more intense during the second period of the disease.

A valuable document on this subject is the paper by Dr. Budd and Mr. Busk, in the 21st vol. of the Med. Chirurg. Transactions, on twenty cases of Cholera, which occurred in October, 1837, on board the Hospital Ship at Greenwich. The circumstances of limitation to this vessel, in which the Cholera had twice existed before, gave much proof of a local reproduction of the virus of the disease.

with the facts regarding Cholera, and is reconcileable with all we know of insect life and reproduction. The ova may remain dormant for long and indefinite periods of time; yet, like the seeds of plants, retain the powers of life, and burst into active existence when circumstances occur to favour the change. One of the most singular facts in natural history is this sudden appearance of insect swarms, with all their instincts complete, in localities where they have not been seen for years before; either brought from unknown distance, or the produce of the ova of former seasons deposited on the spot, in preparation for this later developement.*

And here we again approach to speculations, which, though founded on the most minute forms of existence, have yet a vastness in their obscurity, and in the results to which their solution would lead. I allude to the hypothesis of equivocal generation ;- the question, whether animal or vegetable life

*Though such instances are familiar, even in our own country, I may briefly notice one, somewhat remarkable, and of recent occurrence. In October, 1836, a vast swarm of minute aphides (whether one of the numerous known species was not ascertained) passed over a wide district in Cheshire, Derbyshire, and the southern parts of Lancashire and Yorkshire. The air was so thickly filled with them, that the clothes and faces of persons walking out of doors were completely covered. When getting into the eyes they excited considerable inflammation. The height to which the column reached could not be known. From the best observations in one locality, its superficial extent must have been at least 12 miles in one direction by 5 miles in another; but the detached notices from other places make it certain that the continuous swarm was much more widely spread. No sufficient comparison appears to have been made of local observations to furnish proof as to the rate and direction of movement; but it is worthy of notice, that the town of Manchester was infested by these insects for two or three successive days. Wherever generated, or by whatsoever instinct carried on, there is cause to suppose that the swarm was in transit from one place to another, and possibly brought nearer to the earth by some peculiar state of atmosphere existing at the time.

(for the inquiry equally regards both) are in any case produced without the egg or seed of prior individuals of the species? The result of modern research has been chiefly on the negative side of the question; and the old dogma of 66 omne vivum ab ovo" remains sanctioned to an extent, which could scarcely have been supposed within the scope of human observation. Where, as in the genus Aphis, eight successive generations of females may be fertile without fresh impregnation, it seems needless to seek for other manner of production. Nevertheless, we are still very far from certainty. Instances may be indefinitely multiplied to the effect just stated, and yet it is possible that there are germs of life around us, awaiting developement; or matter so constituted as to be capable of assuming new and various forms of living organization, according to the circumstances present to favour such change. The tendency of all recent inquiry is to bring in life as an element and agent, both in past and exist

*

* The generation of some of the viviparous infusoria seems inexplicable on any other view than this. The question is connected with the equally obscure one as to the immutability of species. In both, the greater familiarity of the idea of successive propagation by ova or seed disguises the fact that their propagation is quite as incomprehensible in itself, as the conversion of the same organic elements, under altered circumstances, into new forms. Notwithstanding this imperfection of our knowledge, the whole subject of organization, whether of animal or vegetable life, affords a signal testimony to the progress of modern science ; — realizing on the one hand some of the poetical imaginations of antiquity; -on the other, by mathematical exactness of observation directed to the most minute forms of matter, laying a basis for discovery, of which the boldest speculator can scarcely yet see the full extent.

The Memoir of MM. Beauperthuy and Roseville, and the observations of Schwann, Cagniard de la Tour, &c., are amongst the most curious of the recent inquiries bearing on this subject; indicating the developement of animal and vegetable life as the probable cause of the various processes of fermentation, putrefaction, &c.

ing phenomena, where before only the relations of inert matter were presumed to exist.

Without entering on this controversy, we find in facts, well ascertained, of the occasional length of time during which the ova of animal life retain their power of reproduction, much that bears on the hypothesis before us. There is reason to presume, that the simpler and more minute the form of organization, the greater is the faculty of thus retaining life in a dormant state. And if complete animal organization, such as that of the infusoria, or the vibriones of wheat, is capable of being restored to vital activity, after long apparent extinction, still more may animal ova be supposed retentive of the principle of life and reproduction. This consideration applies to one of the most curious questions respecting Cholera; viz., how a poison should be thus locally generated, unknown before, but capable, by some means of reproduction, of being diffused over every part of the earth? Difficult though a solution is to every hypothesis, it seems almost impossible on any other than that of animal origin. This furnishes a reason why the disease, existing perchance at some anterior time, may have disappeared for ages; the seed of the pestilence, however, yet remaining, to be called into activity by future contingencies. Or, if we admit the doubtful assertion of some authors, that the same disorder has existed repeatedly in India, though never with equal virulence, no view will so well suggest an explanation, as that which admits on the one side all the instincts of animalcule life; on the other, the varying casualties of place, season, and human communities. Swarms may be evolved in a given locality, and perish, before migration or communication to man have given cause to wider extension of the disease.

The whole subject of this presumed migration will occur as the great difficulty in the hypothesis, seeing that we have

no evidence of any of similar extent in the animal kingdom; and that the manner and duration of insect life are in many respects opposed to one, which involves such variety of country and climate; most of the species known to us appearing to be submitted to geographical distribution, like plants, and the other classes of the animal kingdom. What may be said in reply (even admitting the restriction to insect life, properly so termed) is, that we are singularly ignorant of all that relates to animal migrations in general; — that our knowledge becomes less as we descend to the more minute forms of life; that the facts known respecting the occurrence of insect swarms meet some of these objections; – that different insect species vary infinitely in their habits and capacity for diffusion; that the ova themselves may be widely disseminated; — and that the rapidity of propagation and change, incident generally to this part of creation, if introducing some difficulties, provides the means of obviating others which press upon the argument.*

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Pursuing the hypothesis (still tentatively upon all these points), we reach the question as to the infectious nature of Cholera, and may gain from hence some further argument in removal of the objections just stated. If this point, so warmly contested, be answered in the affirmative, man becomes an agent in the diffusion; and the transit over continents and oceans, otherwise impossible, is brought within comprehension and the analogies of other disease. Much of this controversy might, as I think, have been obviated by a regard to the principles mentioned in a former chapter. It

* Professor Lacordaire of Liège, one of the most recent and able writers on the geographical distribution of insects, records numerous examples showing the influence of particular physical causes upon their distribution or casual diffusion; while yet leaving many instances of diversity and anomaly to which no known explanation will apply.

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