No doubt, the corpuscles of different species––to which, in the last analysis, we reduce animals and plants of every kind and degree—are not identical. Each species has its own structure, its specific energy, its mode of nutrition, its fixed secretions—characteristics, moreover, which vary with circumstances and media. Yet we can point out more than one interesting similarity between certain ones of these species, which seem to discharge quite distinct functions, and hold very unlike stations, in the vast harmony of vital monads. The cells of fruits, when placed in certain conditions, behave, as has been seen, like the cells of brewer's yeast; they both decompose sugar and yield alcohol. We may trace resemblances not less close, as M. Blondeau and M. Pasteur have done, between acetic mycoderms and blood-globules. Both alike serve as carriers of oxygen—the first, for the slow combustion of alcohol; the last, for the slow combustion of the albuminoid matters in animal tissues. It is even likely that there is a principle in mycoderms similar to hemoglobine in the blood-globule, and provided with a special affinity for oxygen. However this may be, comparisons of this kind open a new path for physiology. As that science is definitely summed up in the explanation of existences and processes in the microscopic elements of organs, it is plain that nothing can be more useful to it than the study of these one-celled organisms in which the phenomena are extremely simple, and life is reduced, in a manner, to its primitive factors. It becomes more and more evident that progress in the comprehension of the superior animals is bound, with the very closest ties, to advance in the comprehension of the mechanism of nutrition in the rudimentary units of life, in the smallest beings that it is given us to study.
Now, whence come those organized microscopic corpuscles to which, as we have seen, very many of the alterations of organic matter must be attributed? Upon this great problem, opinions at this day are still very contradictory. Neither patient observations, nor minute experiments, nor profound reasonings, have been wanting; yet some still believe that these little bodies grow, by spontaneous generation, within fermentable liquids, while others assert, and profess to have proved, that they come from germs contained in the air. Certainly, the former opinion involves nothing contradictory nor impossible. Those who reject it by begging the question, in the name of some unknown, mystical doctrine of life, do not even deserve to be listened to in the investigation. It might possibly have occurred that organized beings should be produced, complete at all points, in a medium deprived of organization; yet experiment proves that this does not occur. We must, then, accept the other opinion—the panspermist doctrine—that is to say, we must concede that the germs of microscopic animals and vegetables, with which so many fermentations and putre-