Ehrenberg, in his great work on Infusoria (S. 492-496), has given the most complete history of all the investigations which have taken place on what is called the revivification of rotiferæ. He believes that, in spite of all the means of desiccation employed, the organization-fluid still remains in the apparently dead animal. He contests the hypothesis of “latent life;” death, he says, is not “life latent, but the want of life.” We have evidence of the diminution, if not of the entire disappearance or suspension of organic functions, in the hybernation or winter sleep both of warm and cold-blooded animals, in the dormice, marmots, sand martins (Hirundo riparia) according to Cuvier (Règne animal, 1829, T. i. p. 396), frogs and toads. Frogs, awakened from winter-sleep by warmth, can support an eight times’ longer stay under water without being drowned, than frogs in the breeding season. It would seem as if the functions of the lungs in respiration, for some time after their excitability had been suspended, required a less degree of activity. The circumstance of the sand-martin sometimes burying itself in a morass is a phenomenon which, while it seems not to admit of doubt, is the more surprising, as in birds respiration is so extremely energetic, that, according to Lavoisier’s experiments, two small sparrows, in their ordinary state, decomposed, in the same space of time, as much atmospheric air as a porpoise. (Lavoisier, Mémoires de Chimie, T. i. p, 119.) The winter-sleep of the swallow in question (the Hirundo riparia) is not supposed to belong to the entire species, but only to have been observed in some individuals. (Milne Edwards, Elémens de Zoologie, 1834, p. 543.) As in the cold zone the deprivation of heat causes some animals to fall into winter-sleep, so the hot tropical countries afford an analogous phænomenon, which has not been sufficiently attended to, and to which I have applied the name of summer-sleep. (Relation historique, T. ii. pp. 192 and 626.) Drought and continuous high temperatures act like the cold of winter in diminishing excitability. In Madagascar, (which, with the exception of a very small portion at its southern extremity, is entirely within the tropical zone,) as Bruguière had before observed, the hedgehog-like Tenrecs (Centenes, Illiger), one species of which (C. ecaudatus) has been introduced into the Isle of France, sleep during great heat. Desjardins makes, it is true, the objection that the time of their slumber is the winter season of the southern hemisphere; but in a country in which the mean temperature of the coldest month is 3° Reaumur (6°.75 Fah.) above that of the hottest month in Paris, this circumstance cannot change the three months’ “summer-sleep” of the Tenrec in Madagascar and at Port Louis, into what we understand by a winter-sleep, or state of hybernation.