but it soon resumed its natural standard. They fed on the same aliment, the same drinks, took the same quantity of food, and digestion seemed to be accomplished in the same time. The fæces had the same consistence, the same appearance, and the chyle appeared to have the same character. Nor did the other functions offer any modification.

DUPUYTREN opened several of these dogs some time afterwards, and found no apparent change in the abdominal circulation,-in that of the stomach, epiploon, or liver. The last organ, which appeared to some of the experimenters to be enlarged, did not seem to him to be at all so. The bile alone appeared a little thicker, and deposited a slight sediment.

These circumstances render it extremely difficult to arrive at any theory regarding the offices of this anomalous organ. It is manifestly not essential to life, and therefore not probably inservient to the purposes assigned to it by TIEDEMANN and GMELIN. BOSTOCK properly remarks, that its office must be something of a supplementary or vicarious nature; and this would accord best, perhaps, with the notion of its serving as a diverticulum; the blood speedily passing, after the organ has been extirpated, into other channels. It must be admitted, however, that our knowledge of the function is of a singularly negative and unsatisfactory character.

CLASS III.

FUNCTIONS OF REPRODUCTION, OR GENERATION.

THE functions, which we have been hitherto considering, relate exclusively to the individual. We have now to investigate those, that refer to the preservation of the species; and without which living beings would soon cease to exist. Although these are really multiple, it has been the custom with physiologists to refer them. to one head-generation—of which they are made to form the subordinate divisions.

The function of generation, much as it varies amongst organized bodies, is possessed by them exclusively. When a mineral gives rise to another of a similar character, it is at the expense of its own existence; whilst the animal and the vegetable, produce being after being, without any curtailment of theirs.

The writers of antiquity considered that all organized bodies are produced in one of two ways. Amongst the upper classes of both animals and vegetables they believed the work of reproduction to be effected by a process, which is termed univocal or regular generation; but in the very lowest classes, as the mushroom, the worm, the frog, &c. they conceived that the putrefaction of different bodies, aided by the influence of the sun, might generate life. This has been termed equivocal or spontaneous generation; and is supposed to have been devised by the Egyptians, to account for the swarms of frogs and flies, which appeared on the banks of the Nile after its periodical inundations.

Amongst the ancients the latter hypothesis was almost universally credited. PLINY unhesitatingly expresses his belief, that the rat and the frog are produced in this manner; and, at his time, it was generally thought, that the bee, for example, was derived at times from a parent; but at others from putrid beef.*

The passage of VIRGIL,-in which he describes how the shepherd ARISTEUS succeeded in producing swarms of bees from the entrails of a steer, exposed for nine days to putrefaction,-is probably familiar to most readers:

.

"First in a place, by nature close, they build
A narrow flooring, gutter'd, wall'd and til'd.
In this, four windows are contriv'd, that strike
To the four winds oppos'd, their beams oblique.
A steer of two years' old they take, whose head
Now first with burnish'd horns begins to spread.

"Apes nascuntur partim ex apibus, partim ex bubulo corpore putrefacto."VARRO.

They stop his nostrils, while he strives in vain
To breathe free air, and struggles with his pain.
Knock'd down he dies; his bowels, bruised within,
Betray no wound on his unbroken skin.

Extended thus, in this obscene abode

They leave the beast; but first sweet flow'rs are strew'd
Beneath his body, broken boughs and thyme,
And pleasing cassia, just renew'd in prime.

The tainted blood, in this close prison pent,
Begins to boil, and through the bones ferment.
Then (wond'rous to behold) new creatures rise,
A moving mass, at first, and short of thighs;
Till, shooting out with legs, and imp'd with wings,
The grubs proceed to bees with pointed stings;
And, more and more affecting air, they try
Their tender pinions, and begin to fly.

At length, like summer storms, from spreading clouds
That burst at once and pour impetuous floods-
Or flights of arrows from the Parthian bows,
When from afar they gall embattled foes:-
With such a tempest through the skies they steer,
And such a form the winged squadrons bear."
GEORGIC. Lib. iv.

The hypothesis of equivocal generation having been conceived, in consequence of the impracticability of tracing ocularly the function in the minute tribes of animals, it naturally maintained its ground, as regarded those animals, until better means of observation were invented. The difficulty, too, of admitting regular generation as applicable to all animals, was augmented by the fact, not at first known to naturalists, that many of the lower tribes conceal their eggs, in order that their nascent larvæ may find suitable food.

The existence, however, of evident sexual organs in many of those small species, induced physiologists, at an early period, to believe, that they also might be reproduced by sexual intercourse; but direct proofs were not obtained until the discovery of the microscope; after which the investigations of REDI, VALLISNERI, SWAMMERDAM, HOOKE, REAUMUR, BONNET and others clearly demonstrated, that the smallest insects have eggs and sexes, and that they reproduce like other animals.

In the case of plants, it has been supposed that the growth of the fungi amongst dung, and of the various parasitical plants that appear on putrid flesh, fruit, &c. furnishes facts in support of the equivocal theory; but the microscope, exhibits the seeds of many of these plants, and experiments show that they are prolific. The characters, by which the different species and varieties are distinguished, although astonishingly minute, are fixed; exhibiting no fluctuation, such as might be anticipated, did these plants arise by spontaneous generation, or by the fortuitous concourse of

atoms.

The animalcules, that make their appearance in water, in which

vegetable or animal substances have been infused or are contained, would seem, at first sight, to favour the ancient doctrine. In these cases, however, the species, again, have determinate characters; presenting always the same proportion of parts; and appearing to transmit their vitality to their descendants in a manner not unlike that of animals higher in the scale. The explanation offered by the supporters of the univocal theory for those obscure cases, in which direct observation fails us, is, that their seeds and eggs are so extremely minute, that they can be borne about by the winds; be readily deposited in every situation, and when they find a soil or nidus, favourable to their growth, can undergo development. Thus, the soil, in which alone the monilia glauca flourishes, is putrid fruit; whilst the small infusory animal-the vibrio aceti or vinegar eel, requires, for its growth, vinegar that has been for some time exposed to the air. "That the atmosphere," says Dr. GooD, "is freighted with myriads of insect eggs, that elude our senses; and that such eggs, when they meet with a proper bed, are hatched in a few hours, into a perfect form, is clear to any one who has attended to the rapid and wonderful effects of what, in common language, is called a blight, upon plantations and gardens. I have seen, as probably many, who may read this work, have also, a hopground completely overrun and desolated by the aphis humuli or hopgreen-louse, within twelve hours after a honey-dew, (which is a peculiar haze or mist, loaded with a poisonous miasm,) has slowly swept through the plantation, and stimulated the leaves of the hop to the morbid secretion of a saccharine and viscid juice, which, while it destroys the young shoots by exhaustion, renders them a favourite resort for this insect, and a cherishing nidus for the myriads of little dots that are its eggs. The latter are hatched within eight-and-forty hours after their deposit, and succeeded by hosts of other eggs of the same kind; or, if the blight take place in an early part of the autumn, by hosts of the young insects produced viviparously; for in different seasons of the year, the aphis breeds both

ways.

"Now it is highly probable, that there are minute eggs or ovula, of innumerable kinds of animalcules floating in myriads of myriads through the atmosphere, so diminutive as to bear no larger proportion to the eggs of the aphis than these bear to those of the wren, or the hedge-sparrow; protected, at the same time, from destruction. by the filmy integument, that surrounds them, till they can meet with a proper nest for their reception, and a proper stimulating power to quicken them into life; and which, with respect to many of them, are only found obvious to the senses in different descriptions of animal fluids.

"The same fact occurs in the mineral kingdom; stagnant water, though putrid by distillation and confined in a marble basin, will, in a short time, become loaded on its surface or about its sides with various species of confervas; while the interior will be peopled with

microscopic animalcules. So, while damp cellars are covered with boletuses, agarics and other funguses, the driest brick walls are often lined with lichens and mosses. We see nothing of the animal and vegetable eggs or seeds by which all this is effected; but we know, that they exist in the atmosphere, and that this is the medium of their circulation."

This view of the extraneous origin of the seeds of the confervas, &c. is strongly corroborated by an experiment of SENEBIER. He filled a bottle with distilled water and corked it accurately; not an atom of green matter was produced, although it was exposed to the light of the sun for four years; nor did the green matter, considered as the first stage of spontaneous organization, exhibit itself in a glass of common water, covered with a stratum of oil.

The subject of intestinal worms has been eagerly embraced by the supporters of the doctrine of equivocal generation, who, are of opinion, that the germs need not be received from without; whilst the followers of the univocal doctrine maintain, that they must always be admitted into the system.

The first opinion includes amongst its supporters the names of NEEDHAM, BUFFON, PATRINUS, TREVIRANUS, RUDOLPHI, BREMSER and HIMLY. The latter comprises those who believe in the Harveian maxim, omne vivum ex ovo;-in other words, the majority of the physiologists of the present day.

To support the latter opinion, it has been attempted to show, that the worms, found in the human intestines, are precisely the same as others that have been found out of the body; but the evidence in favour of this position is by no means strong or satisfactory. LINNEUS affirms, that the distoma hepaticum or fluke has been met with in fresh water: the tænia vulgaris,-of a smaller size, however, in muddy springs; and the ascarides vermiculares in marshes and in the putrescent roots of plants. GADD also affirms, that he met with the tænia articulata plana osculis lateralibus geminis in a chalybeate rivulet; UNZER, the tænia in a well; and TISSOT, that he found a tænia, exactly like the human, in a river; whilst LEEUENHOEK, SCHAEFFER, PALMER and others affirm, that they have found the distoma hepaticum in water; but MUELLER, who took extraordinary pains in the comparative examination of the entozoa, that infest the human body, with those that are met with in springs, states that he has frequently detected the planariæ, but never saw one like the distoma hepaticum.

On the other hand, the supporters of the equivocal theory have attempted, with a good deal of success, to show, that a difference is always discoverable between the worms, that are found without and those within the body; but were it demonstrated to a mathematical certainty, that such difference exists, it would not be an invincible argument against the accuracy of the univocal theory; as difference of locality, food, &c. might induce important changes in their corpo