It has been long known that _Bacteria_ and _Torulæ_ are frequently to be found within vegetable cells, taken even from the central parts of plants, whenever these are in a sickly condition or are actually dying. They are apt to exist also within epithelial cells taken from the inside of the mouth; and the frequency and abundance with which such organisms are met with in these cells, is almost in direct proportion to the malnutrition and lack of vital power in the individual who is the subject of observation. Then, again, in persons who have died of adynamic diseases, in the course of twenty-four or thirty-six hours (during warm weather) _Bacteria_ may be found in abundance within the blood-vessels of the brain and of other parts, although no such _Bacteria_ were recognizable in the blood of the individual during life. In these cases we must, in order to account for the presence of the _Bacteria_ and _Torulæ_, either suppose that such organisms, in an embryonic state, are almost universally disseminated throughout the various textures of higher organisms, both animal and vegetal (though they are only able to develop and manifest themselves when the higher organisms, or the parts of them in which the _Bacteria_ or _Torulæ_ are met with, are on the eve of death), or else we must imagine that when the vital activity of any organism, whether simple or complex, is on the wane, its constituent particles (being still portions of living matter) are capable of individualizing themselves, and of growing into the low organisms in question. Just as the life of one of the cells of a higher organism may continue for some time after the death of the organism itself, so, in accordance with this latter view, may one of the particles of such a cell be supposed to continue to live after even cell-life is impossible. Now, to many persons, the latter seems to be a much simpler hypothesis than the former, and one, moreover, which is more in accordance with known facts. People’s views, however, on this subject are likely to be much influenced by their notions as to the possibility of _Bacteria_ arising by a process of Archebiosis. Although some may be inclined to accept the doctrine of Heterogenesis, the same persons, being “vitalists,” may not readily believe in the doctrine of Archebiosis, because this implies the vivification of dead matter--the conversion of not-living elements into a living combination. Those, however, who do believe in Archebiosis will--if the necessary evidence be forthcoming--all the more readily yield their assent to the doctrine of Heterogenesis, because it is a much less novel thing to have to believe in the mere transformation of living matter, than in the possibility of its origin _de novo_. Evidence of a tolerably satisfactory nature, however, is forthcoming, which may speak independently in favour of the doctrine of _Heterogenesis_.