But, without presuming to discuss a question which leads us into the very arcana of chemistry, the present state of speculation upon the modus operandi of the yeast plant in producing fermentation is represented, on the one hand, by the Stahlian doctrine, supported by Liebig, according to which the atoms of the sugar are shaken into new combinations, either directly by the Torulae, or indirectly, by some substance formed by them; and, on the other hand, by the Thenardian doctrine, supported by Pasteur, according to which the yeast plant assimilates part of the sugar, and, in so doing, disturbs the rest, and determines its resolution into the products of fermentation.  Perhaps the two views are not so much opposed as they seem at first sight to be.

But the interest which attaches to the influence of the yeast plants upon the medium in which they live and grow does not arise solely from its bearing upon the theory of fermentation.  So long ago as 1838, Turpin compared the Torulae to the ultimate elements of the tissues of animals and plants—­“Les organes elementaires de leurs tissus, comparables aux petits vegetaux des levures ordinaires, sont aussi les decompositeurs des substances qui les environnent.”

Almost at the same time, and, probably, equally guided by his study of yeast, Schwann was engaged in those remarkable investigations into the form and development of the ultimate structural elements of the tissues of animals, which led him to recognize their fundamental identity with the ultimate structural elements of vegetable organisms.

The yeast plant is a mere sac, or “cell,” containing a semi-fluid matter, and Schwann’s microscopic analysis resolved all living organisms, in the long run, into an aggregation of such sacs or cells, variously modified; and tended to show, that all, whatever their ultimate complication, begin their existence in the condition of such simple cells.

In his famous “Mikroskopische Untersuchungen,” Schwann speaks of Torula as a “cell;” and, in a remarkable note to the passage in which he refers to the yeast plant, Schwann says:—­

“I have been unable to avoid mentioning fermentation, because it is the most fully and exactly known operation of cells, and represents, in the simplest fashion, the process which is repeated by every cell of the living body.”

In other words, Schwann conceives that every cell of the living body exerts an influence on the matter which surrounds and permeates it, analogous to that which a Torula exerts on the saccharine solution by which it is bathed.  A wonderfully suggestive thought, opening up views of the nature of the chemical processes of the living body, which have hardly yet received all the development of which they are capable.