rently powerful argument has been based by believers in
spontaneous generation. Similar arguments have been used by the
opponents of the germ theory of epidemic disease, who have
triumphantly challenged an appeal to the microscope and the chemist's
balance to decide the question. Such arguments, however, are founded
on a defective acquaintance with the powers and properties of matter.
Without committing myself in the least to De la Rive's notion, to the
doctrine of spontaneous generation, or to the germ theory of disease,
I would simply draw attention to the demonstrable fact, that, in the
atmosphere, we have particles which defy both the microscope and the
balance, which do not darken the air, and which exist, nevertheless,
in multitudes sufficient to reduce to insignificance the Israelitish
hyperbole regarding the sands upon the sea-shore.

*****

The varying judgments of men on these and other questions may perhaps
be, to some extent, accounted for by that doctrine of Relativity which
plays so important a part in philosophy. This doctrine affirms that
the impressions made upon us by any circumstance, or combination of
circumstances, depend upon our previous state. Two travellers upon
the same height, the one having ascended to it from the plain, the
other having descended to it from a higher elevation, will be
differently affected by the scene around them. To the one nature is
expanding, to the other it is contracting, and impressions which have
two such different antecedent states are sure to differ. In our
scientific judgments the law of relativity may also play an important
part. To two men, one educated in the school of the senses, having
mainly occupied himself with observation; the other educated in the
school of imagination as well, and exercised in the conceptions of
atoms and molecules to which we have so frequently referred, a bit of
matter, say 1/50000th of an inch in diameter, will present itself
differently. The one descends to it from his molar heights, the other
climbs to it from his molecular lowlands. To the one it appears
small, to the other large. So, also, as regards the appreciation of
the most minute forms of life revealed by the microscope. To one of
the men these naturally appear conterminous with the ultimate
particles of matter; there is but a step from the atom to the
organism. The other discerns numberless organic gradations between
both. Compared with his atoms, the smalles