ons of fixed stars in the sky, the distribution of land and water
on the globe, opposite seasons in opposite hemispheres.

All these cases of Co-existence (except the geometrical) present the
problem of deriving them from Causation; for there is no general Law of
Co-existence from which they can be derived; and, indeed, if we conceive
of the external world as a perpetual redistribution of matter and
energy, it follows that the whole state of Nature at any instant, and
therefore every co-existence included in it, is due to causation issuing
from some earlier distribution of matter and energy. Hence, indeed, it
is not likely that the problems of co-existence as a whole will ever be
solved, since the original distribution of matter is, of course,
unknown. Still, starting with any given state of Nature, we may hope to
explain some of the co-existences in any subsequent state. We do not,
indeed, know why heavy bodies are always inert, nor why the chemical
elements are what they are; but it is known that "the properties of the
elements are functions of their atomic weight," which (though, at
present, only an empirical law) may be a clue to some deeper
explanation. As to plants and animals, we know the conditions of their
generation, and can trace a connection between most of their
characteristics and the conditions of their life: as that the teeth and
stomach of animals vary with their food, and that their colour generally
varies with their habitat.

Geometrical Co-existence, when it is not a matter of definition (as 'a
square is a rectangle with four equal sides'), is deduced from the
definitions and axioms: as when it is shown that in triangles the
greater side is opposite the greater angle. The deductions of theorems
or secondary laws, in Geometry is a type of what is desirable in the
Physical Sciences: the demonstration, namely, that all the connections
of phenomena, whether successive or co-existent, are consequences of the
redistribution of matter and energy according to the principle of
Causation.

Coincidences of Co-existence (Group (3)) may sometimes be deduced and
sometimes not. That 'nauseous insects have vivid coloration' comes under
the general law of 'protective coloration'; as they are easily
recognised and therefore avoided by insectivorous birds and other
animals. But why white tom-cats with blue-eyes should be deaf, is (I
believe) unknown. When co-existences cannot be derived from causation,
they can only