and plant groups constantly appearing as series of parallel
modifications of similar and yet different primary forms. In the
living world, facts of this kind are now understood to mean evolution
from a common prototype. It is difficult to imagine that in the
not-living world they are devoid of significance. Is it not possible,
nay probable that they may mean the evolution of our 'elements' from a
primary undifferentiated form of matter? Fifty years ago, such a
suggestion would have been scouted as a revival of the dreams of the
alchemists. At present, it may be said to be the burning question of
physico-chemical science.

In fact, the so-called 'vortex-ring' hypothesis is a very serious and
remarkable attempt to deal with material units from a point of view
which is consistent with the doctrine of evolution. It supposes the
ether to be a uniform substance, and that the 'elementary' units are,
broadly speaking, permanent whirlpools, or vortices, of this ether,
the properties of which depend on their actual and potential modes of
motion. It is curious and highly interesting to remark that this
hypothesis reminds us not only of the speculations of Descartes, but
of those of Aristotle. The resemblance of the 'vortex-rings' to the
'tourbillons' of Descartes is little more than nominal; but the
correspondence between the modern and the ancient notion of a
distinction between primary and derivative matter is, to a certain
extent, real. For this ethereal 'Urstoff' of the modern corresponds
very closely with the prhôtê hylê of Aristotle, the _materia prima_ of
his mediæval followers; while matter, differentiated into our
elements, is the equivalent of the first stage of progress towards the
heschhatê hylê, or finished matter, of the ancient philosophy.

If the material units of the existing order of nature are specialised