wise than by 'spontaneous generation,' Spallanzani was compelled to assume (a) that the unknown germs whose existence had been postulated, notwithstanding their animal origin, were of the nature of seeds rather than eggs, because they were capable of undergoing desiccation with impunity-such ability to survive desiccation conferring upon them the greater power of resisting heat which characterizes seeds. Nay, further (b), although no seeds could be shown to be able to resist the influence of boiling water, Spallanzani assumed that these unknown seed-like germs might be able to do so.* Thus alone was he able to continue in the Panspermatist faith-on the strength of these hazardous assumptions only, could he refuse assent to the probability of a germless origin of living matter, more or less after the fashion suggested by Needham and others. We may, therefore, now consider how far the progress of science has tended to confirm or reverse the hypothesis by which Spallanzani sought to shelter * He had only met with a few seeds which had resisted a momentary exposure in dry sand to a temperature of 212° F. But seeing that not one of the numerous seeds with which he had experimented had been able to survive a similar momentary exposure to boiling water, he had no real warrant for supposing that the germs in question would be able to do so. Spallanzani, in short, here committed the error of arguing that what had occurred in dry sand might occur in water-even though his own experience had not supplied him with a single instance of survival of egg or seed after it had been even momentarily scathed by boiling water. himself, and ward off the conclusions of his opponent. He saw fully, and had frankly admitted, that there was but one means of escaping from Needham's conclusions. But, were these means legitimate? (a) Although it is doubtless true that the superior dryness of seeds does enable them to resist the influence of heat longer than moist eggs are able to do, and therefore also enables them apparently to resist for a brief period a temperature notably higher than would have proved fatal to them had they been in a moist state-it is altogether another question when we have to decide whether the naturally moist Bacteria or their germs are really endowed with this seed-like property of developing after desiccation. To maintain his Panspermism in the face of his own experiments, Spallanzani was compelled to assume that the germs of the lower Infusoria do possess this potentiality. Modern science, however, declares that they have no such property. We are told most unreservedly by Professor Burdon Sanderson,† not only that "the germinal particles of microzymes [Bacteria] are rendered inactive by thorough drying without the application of heat," that is, by mere exposure to air for two or three days at a temperature of * See p. 142. + "Thirteenth Report of the Medical Officer of the Privy Council," p. 61. 104° F., but also that, "fully-formed Bacteria are deprived of their power of further development by thorough desiccation." Thus is the most important assumption made by Spallanzani swept away, and with it all the strength that his position may have appeared to possess. His followers cannot hope to save their germs from the full action of heat, however much they may wish to do so (and) there are strong signs that they are thus influenced) by assuming the pre-existence of a protective desiccation. Are they not told, on what is to them the unquestionable authority of Professor Sanderson, that such desiccation would be in itself destructive? (b) We are left, therefore, face to face with only one other question. Has the progress of science, it may be asked, since the time of Spallanzani, in any way tended to strengthen the possibility that Bacteria germs, or any forms of living matter in the moist state, can resist the destructive action of boiling water, even for two or three minutes? To this question a negative answer may be unreservedly given. The progress of science has, on the contrary, shown that such a supposition becomes more and more improbable when judged by the light of all uncomplicated investigations bearing on the subject. To these results of modern research I must now call the reader's attention. In the first place the specific question with which we are more immediately concerned, as to the thermal death-point of Bacteria and their germs, has itself been answered by most decisive experiments. As the writer has elsewhere already shown,* all direct experimentation on this subject leads to the conclusion that Bacteria and their germs, whether visible or invisible, are killed by a brief exposure to a heat of 140° F. in the moist state. Thus Dr. Sanderson's experiments having proved that the germs of these organisms are, as regards their ability to withstand desiccation, related to eggs rather than to seeds, the writer's own experiments tend still further to strengthen this resemblance by showing that these Bacteria germs (like the eggs with which Spallanzani experimented) are invariably killed at a temperature of about 140° F. Although, therefore, my experiments are not favourable to Spallanzani's assumptions, they are entirely in accordance with his experiments. The thermal death-point ascertained by him for the eggs of Insects and of Batrachia agrees almost exactly with that which I have established for Bacteria germs-although at the time my own experiments were made I was unaware of these particular results obtained by Spallanzani.† In the two papers which precede this. + Up to that time I had read his earlier work entitled, "Nouvelles Is there, then, anything in this fact concerning Bacteria and their germs at all at variance with what we might have been led to expect, judging from our knowledge of the capacity for resisting heat shown by other kinds of living matter? Here again a negative answer may be unreservedly given. The grounds for this opinion must, however, be set forth; and in dealing with this important question I will range what I have to say under the following heads: (1) The results obtained by many other investigators working quite independently of one another (and in many cases also without distinct reference to the Origin of Life question) all go to show that different kinds of living matter are killed, when heated in the moist state, at or below the temperature of 140° F. (2) The only known exceptions to this rule, furnished by organisms in hot springs, are cases of a special kind differing altogether from those with which we are at present concerned : though even here it is found that all such organisms perish at temperatures short of the boiling point. (3) Our knowledge as to the thermal death-point of Living Organisms and of units of Living Matter is remarkably harmonious, and is in accordance there Recherches sur les Découvertes Microscopiques, et la Génération des Corps Organisés," &c., and not the one from which I have just been quoting. |