know that under
certain conditions the old atomic associations break up, and new ones
are formed. But though the processes are hidden, the results are
manifest in the changes which are brought about upon the masses of
material which are subjected to the altering conditions. Gradually the
chemists of our day are learning to build up in their laboratories
more and more complicated compounds; already they have succeeded in
producing many of the materials which of old could only be obtained by
extracting them from plants. Thus a number of the perfumes of flowers,
and many of the dye-stuffs which a century ago were extracted from
vegetables, and were then supposed to be only obtainable in that way,
are now readily manufactured. In time it seems likely that important
articles of food, for which we now depend upon the seeds of plants,
may be directly built up from the mineral kingdom. Thus the result of
chemical inquiry has been not only to show us much of the vast realm
of actions which go on in the earth, but to give us control of many of
these movements so that we may turn them to the needs of man.

Animals and plants were at an early day very naturally the subjects of
inquiry. The ancients perceived that there were differences of kind
among these creatures, and even in Aristotle's time the sciences of
zooelogy and botany had attained the point where there were
considerable treatises on those subjects. It was not, however, until a
little more than a century ago that men began accurately to describe
and classify these species of the organic world. Since the time of
Linnaeus the growth of our knowledge has gone forward with amazing
swiftness. Within a century we have come to know perhaps a hundred
times as much concerning these creatures as was learned in all the
earlier ages. This knowledge is divisible into two main branches: in
one the inquirers have taken account of the different species, genera,
families, orders, and classes of living forms with such effect that
they have shown the existence at the present time of many hundred
thousand distinct species, the vast assemblage being arranged in a
classification which shows something as to the relationship which the
forms bear to each other, and furthermore that the kinds now living
have not been long in existence, but that at each stage in the history
of the earth another assemblage of species peopled the waters and the
lands.

At first naturalists concerned themselves on