Suspension of vital action under other cir cumstances.—We have next to consider those cases in which vitality is rendered for a time dormant, by causes originating in the system itself, rather than by the withdrawal of external stimuli. Under this head we may place all those phenomena to which the name of hyber nation is usually given ; but which, as will presently be seen, cannot be appropriately de signated by that term. The greater number of plants indigenous to temperate climates un dergo an annual series of phases, in which their vegetative processes exhibit every grada tion from a torpor apparently complete to the most surprising activity. In many, indeed, this series of phases constitutes the whole of life; the individual ceasing to exist as soon as it has been once performed, and a new genera tion called into existence. In many more, a total suspension of activity appears to take place, as may be observed in plants whose stems die annually, whilst the roots retain their vitality. This condition exactly resembles that of certain animals which pass the winter in a state of profound torpor. In those, however, whose stems are woody and persistent, vital action does not seem to be completely checked even by a frosty atmosphere ; as late experi ments show that a movement of sap takes place, though to a trifling degree, in the depth of winter. And, lastly, in evergreen plants, these changes of condition are less complete ; the activity of the vegetative processes being diminished by the partial withdrawal of their appropriate stimuli, but not being altogether suspended. Now although it is unquestion able that this series of changes is greatly lu enced by the successive alterations in the ex ternal conditions of the beings, which the revolution of the seasons induces, it does not admit of doubt that originally dependent on the peculiar constitution of the organism, by which a periodical diminution of its activity is occasioned. For nothing will prevent a plant from shedding its leaves nearly at its usual time ; and although by artificial heat, or by removal to a warmer climate, a new crop may be brought out within a short interval, this can only be effected by keeping in a state of activity the processes which ought to be at rest, so that an injurious influence is exerted on the general system like that which results from artificially prolonged watchfulness in animals. When a plant is reduced, by the periodical decay of its stem and leaves, to the state of a bulb or root, it seems almost to revert to that remarkable condition already described as peculiar to seeds; the vitality of the structure being capable of remaining dormant for a considerable time, and of being then aroused into full activity by the appropriate stimuli. We are not aware of any authentic facts which fix the limit to the dura tion of this condition. Instances have been related of the growth of bulbs unrolled from the envelopes of Egyptian mummies ; but there is reason to believe that deception has been practised on this point upon the too-ready credulity of travellers. However, there can be no doubt that, under favourable circumstances, bulbs and roots may be preserved for many years ; the cond itions necessary for this object being such as neither excite the vitality of the structure to action, nor occasion the disintegration of the latter and the consequent loss of its properties.
The animal kingdom presents us with condi tions very analogous to those just alluded to. In a large proportion of those inhabiting tem perate climates, there is a periodic diminution of vital activity during the colder part of the year; but this, in the higher tribes at least, scarcely amounts to an absolute suspension, since the circulation, and the functions of nu trition and secretion which depend on it, are carried on,though feebly. (See HYBER N Knox.) It is easy to understand why this must be the case. The softer portions of the animal frame, which are most concerned in the processes of organic life, are not periodically cast off and renewed like the corresponding parts of plants ; and, if their integrity were not maintained by the circulation of nutritious fluid during their inactive state, their normal constitution would be soon affected by their proneness to de composition, and their peculiar properties be consequently lost. Amongst cold-blooded ani
mals, however, we find instances of more com plete suspension of vital actions, which may even be prolonged for a considerable period. Thus, Spallanzani kept frogs, salamanders, and snakes, in a torpid state, in an ice-house, where they remained three years and a half, and rea dily revived when again exposed to the influ ence of a warm atmosphere. Insects, in their pupa state, may be regarded as analogous to plants reduced to bulbs. Although the dura tion of this torpid condition is ordinarily deter minate for each species, and although some changes occur during its continuance which scarcely warrant its in characterising the state as one of entire inactivity, there are some in stances which prove that it may be prolonged for an almost indefinite period, under particular circumstances. The degree of temperature to which pupae are exposed seems to have the same kind of influence over them as on the eggs of insects. Thus Iteaumur found that pupae, which would not naturally have been disclosed until May, might be caused to un dergo their metamorphosis in a fortnight during the depth of winter, by the influence of artifi cial heat; and, on the other hand, that their change might be delayed a whole year beyond its usual time, by the prolonged influence of a cold atmosphere. We can scarcely imagine, however, that temperature is the sole agent in accelerating or retarding the final metamor phosis. If the caterpillar of Papilio Mocha on, one of those which has annually a double brood, becomes a pupa in July, the butterfly will appear in thirteen days; if not until Sep tember, it will not make its appearance until the June following, that is, not in less than nine or ten months. Here it is evident that the torpor has been prolonged from some cause in herent in the system itself, for the purpose of preventing the disclosure of the butterfly at too early a period of the season. A still more cu rious proof of this tendency to prolonged tor pidity during the pupa state is the following. If a number of the pupa: of the Erlogaster lanestris, a moth whose larvae are common on the blackthorn in Susie, be selected at the same time, and placed in the same circum stances, the greater number of them will dis close the perfect insect in the February follow ing; some not until the February of the year ensuing; and the remainder not before the same month in the third year. The same has been observed of the Arclea mendica, of which thirty-six pupa., grown from eggs laid by the same parent, produced twelve perfect insects in each of the three following seasons.* Thefinal cause of this curious tendency may be, as sur mised by Mr. Kirby, to secure the race from being cut off by unfavourable seasons, or by some extraordinary increase of its natural ene mies. But its efficient cause can only be looked for in some modification of the properties of the organism analogous to that which the phenomena of hybernation in other animals. The same periodicity, manifesting itself, not in obedience to a diminished temperature, but at the season of greatest heat, is observed in tro pical climates. Many tribes of insects in the torrid zone seem to retire to places of retreat during the parching droughts of summer, and make their appearance again during the rainy season, when vegetation is in the highest luxu riance. We here trace the same beautiful adaptation of the phases of animal and ; table life as in the former instances ; efficient cause which induces these cl must be different.