VI. VITALITY IN A DORMANT On INACTIVE CONDITION.—There are many organised beings at particular periods of whose existence all vital action seems to be suspended; and this may result either from the absence of the sti muli necessary to maintain it, or from some change in the organism itself, by which it be comes for a time less capable of responding to these stimuli. NVlien vital action is suspended from the deficiency of external stimuli, one of two things must happen ; either the vitality of the organism will be destroyed by the disin tegration of its tissues; or it may be preserved in consequence of the absence of those agents which ordinarily excite decomposition. The occasional suspension of vital action from a change in the organism itself, appears usually to result from a general law of periodicity, which affects, more or less, all organised beings, producing the phenomena of sleep, hybernation, &c.; but it may also arise from particular causes operating within the system, as in syncope. Each of these cases will now be separately considered.
Dormant vitality of seeds, eggs, 4c.—The condition of organised beings of which we have first to treat—that in which vital action is sus pended from the absence of the stimuli ne cessary to maintain it, and vitality never theless preserved—is manifested in the most remarkable manner by the reproductive germs which are periodically separated from plants and animals, and which are endowed with the power of developing themselves into new indi viduals when the requisite conditions are sup plied to them. In the lowest classes of each kingdom, it would appear that these germs are hherated from the parent unprovided with any means for thecontirmance of their development ; and that from the first, therefore, they rely upon the surrounding elements for all the conditions of their active existence. It is beautifully pro vided that, in proportion to the probable defi ciency of some of these, should be the tenacity with which the apparently lifeless germs re , tain their vitality. The sporules of the fungi, which can only subsist on decaying organised matter, seem universally diffused through the atmosphere, and ready to vegetate with the most extraordinary rapidity whenever a fitting nidus is afforded for their development. This, at least, appears the only feasible mode of ex plaining their appearance in the forms of mould, 1 mildew, &c. on all decaying surfaces ; and that there is no improbability in the suppo sition itself is shown by the estimate of Fries, who states that a single individual of reti cularia maxima will emit above 10,000,000 of these germs, so minute as when collected to be scarcely visible to the naked eye, rather re sembling thin smoke, and so light as to be wafted by every movement of the atmosphere, so that, he remarks, " it is difficult to conceive a place from which they can be excluded."
It seems more than probable that in a similar manner is to be explained the appearance of infusorial animalcules in all situations adapted to their existence ; and that their germs are constantly and universally diffused through the air, ready to commence the active exercise of their dormant properties whenever they meet with the stimuli to their development afforded by warmth, moisture, and decomposing organic matter.* We have no means of ascertaining the length of time during which this dormant vi tality may be preserved. It would be difficult to assign a limit to it, since it is scarcely con ceivable that any change can occur in the struc ture of these minute desiccated points which they do not undergo during the first few hours of their aerial residence ; and we have no reason to believe that vitality can be de stroyed without change of structure. With re gard to the seeds of phanerogsmic plants, we have more certain evidence, and this of a very interesting character. It is to be remarked, however, that in them, as in the eggs of higher animals, there is, besides the germ it self, a reservoir of nutriment supplied by the parent, which enables the germ to continue its development up to the point at which it be comes fit to maintain its own existence, with out any other than the ordinary assistance of vital stimuli. The germination of a seed, for example, requires only warmth, moisture, and the access of air, and is further accelerated by the absence of light andthe hatching of an egg is dependent only on a temperature more or less elevated and the presence of air. hence the necessity for so great a tenacity of vitality as that possessed by the germs of the simpler classes does not exist, and although under favourable circumstances the vitality of seeds may be prolonged for an almost indefinite period, they are more susceptible of the inju rious influence of external agents, and their fertility is destroyed by changes of condition which would have no effect in the former case ; whilst the eggs of animals appear still less tena cious of vitality, although in a few instances capable of retaining it for some time, even under considerable disadvantages, as will be presently noticed.