the vegetable tribes are accomplished;
through their mutual relations with the atmospheric air, plants and
animals are interbalanced, and neither permitted to obtain a
superiority. The condensation of carbon from the air and its inclusion
in the strata constitute the chief epoch in the organic life of the
earth giving a possibility for the appearance of the hot-blooded and
more intellectual animal tribes. That event was due to the influence of
the rays of the sun.

Passing from inorganic to organic forms, our author remarks that their
permanence is altogether dependent 'on the invariability of the material
conditions under which they live. Any variation therein, no matter how
insignificant it might be, would be forthwith followed by a
corresponding variation in the form.' At this point we are brought to
the far-famed 'development theory,' which, since the publication of the
'Vestiges of Creation,' has been the scientific battle field of the
naturalists of the world. Professor Draper is, of course, a firm
adherent of this theory. He continues:

'The present invariability of the world of organization is the
direct consequence of the physical equilibrium, and so it will
continue as long as the mean temperature, the annual supply of
light, the composition of the air, the distribution of water,
oceanic and atmospheric currents, and other such agencies, remain
unaltered; but if any one of these, or of a hundred other incidents
that might be mentioned, should suffer modification, in an instant
the fanciful doctrine of the immutability of species would be
brought to its true value. The organic world appears to be in
repose, because natural influences have reached an equilibrium. A
marble may remain forever motionless upon a level table; but let
the surface be a little inclined, and the marble will quickly run
off. What should we say of him who, contemplating it in its state
of rest, asserted that it was impossible for it ever to move?

'When, therefore, we notice such orderly successions, we must not
at once assign them to a direct intervention, the issue of wise
predeterminations of a voluntary agent; we must first satisfy
ourselves how far they are dependent upon mundane or material
conditions, occurring in a definite and necessary series, ever
bearing in mind the important principle that an orderly sequence of
inorgan