tion, and are classed, according to the manner in which it is performed, into three genera, which inhabit infusions, and may easily be distinguished. The bacteria are rigid and balanced. as of one piece; the vibrions are flexible, and endowed with a wormlike movement; and the spirilla resemble corkscrews, and move in spirals, as a screw in a nut.

The vegetable kingdom furnishes microscopic fungi of the families of the mucidines, torulacci, and moulds, whose essential characteristics are like those of the ordinary mushroom. This is known to be a subterranean plant which occurs in manure beds, where it forms a close web of white threads, called by botanists mycelium. It is endowed with a surprising vitality. We may dry it, heat it to the boiling point, keep it for years, and revive it again by restoring it to the conditions of temperature, moisture, etc., in which it first took life. When mature it develops rapidly, often in a night, an exterior eatable excrescence, commonly regarded as the fungus itself, but which is considered by the botanist as simply the organ of fructification. This bears the germs or spores, fine light grains, which detach themselves, are blown away, and sow themselves anew. This evolution, well known in the common mushroom, is performed with at least equal fruitfulness in the microscopic fungi. If, for example, we leave a piece of bread in the moist air, a mycelium which can be sown will establish itself and creep through the tissue and push up its exterior stems. To the naked eye, and in common language, this is breadmould; the microscope shows it to be composed of branching stems, bearing spores, which detach themselves and fly away at maturity. It is the penicilium glaucum, a fungus as well known and classified with as much precision as the eatable agaric, which is sown in the Roquefort cheeses and developed as the green mould, a characteristic and special merit of that esteemed viand. The blight which destroys the leaves of fruit-trees, the oidium of the grapevine, are fungi of the same family, and related to those which cause the potato disease. Easy to sow, multiplying infinitely, resisting every remedy, they attack everything; wheat as rust, rye as ergot, larvæ, living insects, silkworms, etc. Each chooses the station which suits it best, where it can find its special nourishment, from which it generally receives its name.

Still lower in the vegetable kingdom are forms more mysterious on account of the mission which is reserved for themthe ferments. The most studied and best known is the yeast plant. To the naked eye it is a yellowish pulp, a kind of dregs, which is formed in the making of beer. If we put a little of it in a liquor containing considerable sugar, a little nitrogenized matter, and phosphates, it increases like a plant in rich soil. It is really a fungoid plant, (torula cerevisia,) which under the microscope shows a mass of roundish globules without interior details. Observing one of them, we may see a bubble rise to the surface and grow till it resembles the primitive globule, and is reproduced like it. Thus begins and continues gradually by germination the increase of this living pulp. This plant accomplishes one of the most marvelous phenomena which it has been given to chemists to observe. It decomposes the sugar, resolving it into carbonic acid, which escapes in bubbles, and alcohol which remains in the liquor. Beer is thus made by a special chemical action due exclusively to the vital process of a microscopic body, to a function as necessary to it as respiration to us. Take away the sugar and it perishes, as we would without air. This ferment is not the only one known. Chemistry reveals many analogous species, each causing special chemical actions which transform by ferments a large mass of natural substances. These bodies play an important part in terrestrial life, for their number is immense, and they are multiplied beyond all conception when they meet the requisite conditions.

The reader now knows all that is necessary concerning those microscopic beings in which nature so abounds, and whose study is so necessary on account of the service they render us and the evils they bring upon us. The question of their generation is far from being a question of mere curiosity, and its solution is very important, as we must learn how to increase and destroy them. We must first describe the principal circumstances under which they appear.

Let us macerate in pure water the leaves or stems of some plant, a handful of hay, for example, or some animal substance, as milk, blood, urine, or any organic tissue. After carefully filtering it, let it be put in a vessel and covered, or even corked, taking care, however, to leave the air in contact with the

liquid. In a couple of days, if the requisite temperature is preserved, the surface will be covered with a thin vail, which will thicken in time and become a consistent film. This will be peopled with bacteria, vibrions, and spirilla; it will swarm with monads and colpods; it will serve as the soil for a forest of mould-plants. Not every solution will produce all these creatures; but they will be found in those which are adapted to their wants. The most remarkable circumstance is, that in liquors will appear the ferments which can decompose them. Thus the yeast-plant appears whenever there are sugar, nitrogenized matter, and phosphates, and the mycoderm of vinegar in wine to change it into acetic acid; and in general every species presents itself wherever it can exercise its special function. Such experiments have been varied in every possible manner; the results have always been constant, and may be summed up as follows: Every organic substance kept moist in contact with atmospheric air, at a temperature of from 15° to 25° centigrade, will be inhabited naturally, internally and externally, by infusoria or moulds, whose species will vary with the substance, and of whose origin we are ignorant.

In all discussion it is an important point for the contestants to agree upon the fundamental facts. This condition is realized in the present case. The law we have enunciated is indisputable; no one thinks of disputing it; but physiologists, though agreeing as to the phenomena, differ in the.explanation. Some reason as follows: When we take away any part from a vegetable or an animal, that part ceases to live. Its organic elements become free, and the part of life which they possessed is released from the collective life of the whole, and becomes individual. It is employed in vitalizing vibrions, ciliated infusoria, or fungi, and these beings, owing their birth to the decomposition of an anterior life, live separately under favoring circumstances. This idea, admitted by Buffon, and agreeing with the opinion recently announced by M. Fremy in the Academy of Sciences, asserts that life in one form may be continued in another, and is well expressed by the term heterogeneity. Those who hold to this theory do not then suppose, as is generally thought, that life can spring from nothing; in fact, they regard that as inadmissible; they simply assume the possibility of the parceling out of a life at the moment of FOURTH SERIES, VOL. XVII.—34

extinction into other existences just beginning, which would be, as it were, its small change. It must be acknowledged that there is nothing in such a theory contrary to sound philosophy.

Other physiologists propose a different explanation, more in accordance with the general laws of nature. These microscopic organisms, they say, like superior beings, receive life from ascendants which they resemble, and transmit it without change to those which come after them. If we cannot discover their generative organs, or witness their birth, it is because they are so small and nimble as to escape our observation. They are so fruitful, and their germs so retentive of life and numerous, that they are diffused and accumulate everywhere. When there are found in a given place the circumstances which favor the development and support of certain species, their germs, of which there is no lack, are there, ready to burst, live, and fructify. The term panspermy, expressing the universal diffusion of the germs, is applied to this theory. It is not less reasonable than that of heterogeneity. Both opinions bear the same relation to religious principles and the data of philosophy, which cannot pretend to discover the exact solution of the problem. The question has been brought to the surer tribunal of experiment.

It must be admitted that the two theories do not present themselves with the same chances of success. There is a characteristic difference in the methods of proof upon which they must rely. Heterogeneity implies a negation, and can only invoke negative proofs. It must prove, first, that there are no germs either in the air or in putrescible liquors; in the second place, that we can kill, in the air and in organic matter, the germs which we may suppose to be there, without destroying the spontaneous fecundity of the putrescible solutions. It is enough that an experimentist be unskillful and fail in finding or killing the germs, for him to believe that he may conclude that there are none. Thus the panspermatists can always say to their adversaries, You can neither discover nor kill the germs, because you are not skillful enough; and the heterogenists, who would seem to be wrong even if they were right, are driven to argumentation, which offers but a weak support, or to assume the attitude of negation, which prudence condemns,

but which has not dismayed some eminent and earnest men. Among these, in France, in the first rank from age, reputation, and talent, is M. Pouchet, corresponding member of the Institute, director of the museum of Rouen, author of numerous remarkable labors in micrography. Besides him are Messrs. Joly and Musset, professors of the faculty of Toulouse. M. Joly has during the last year expounded, in a lecture at Paris, before an interested audience, the doctrine of heterogeneity. Received at first with sympathetic curiosity, he had the gratification of shaking the convictions of some, and of reporting at Toulouse ardent adhesions of a part of the scientific press.

The task of the panspermatists seems more laborious; it is incomparably more delicate. They must show that there are always germs in the air, upon all the bodies that have been exposed to it, in all standing solutions, in every country and place. They are required to show these germs, to plant them, and to gather a harvest of like bodies to those which have produced them. To complete the proof, they must be able, by suppressing all the germs, to make sterile spontaneously putrescible solutions. If they succeed in accomplishing all this, we must submit to the authority of an irresistible demonstration. We shall soon see in what degree this task has been accomplished. It was first undertaken by M. Pasteur. M. Coste, a weighty authority, seconds him. Messrs. Milne Edwards and Chevreul, and a large number of scientific men, have also adopted panspermatic views. Thus the facts are admitted without dispute, the question and the issues are plainly expressed. On both sides are men of high talent and equal sincerity and courtesy; under these conditions the contest has been carried on before an interested and curious public.

The debate was opened by M. Pasteur in February, 1860, in this wise: The atmosphere is never pure; it is defiled by a multitude of minute bodies, which are kept up by its resistance, and are displaced by the slightest breeze. This can be easily proved by introducing a ray of light into a darkened chamber, when the illuminated particles become visible. They are innumerable, always in motion, and they penetrate everywhere. If there are germs in the air, they certainly form part of this floating world, and may be collected by filtering the air