organized beings, just as much as their introduction at first, belong
to the will and plan of God. Revelation opposes no obstacle to any
scientific investigation of the nature and method of this plan, nor
does it contemplate the idea that any discoveries of this kind in any
way isolate the Creator from his works. Farther, inasmuch as God is
always present in all his works, one part of his procedure can
scarcely be considered an "intervention" any more than another.

10. As an illustration of the hypothetical condition of this subject,
and of the views which may be taken as to its details, I quote from a
memoir of my own certain conclusions with reference to the origin of
the species of land plants which are found in the older geological
formations. The conclusions stated are at the end of a detailed
consideration of these plants and the circumstances of their
occurrence:

"(1.) Some of the forms reckoned as specific in the Devonian and
Carboniferous formations may be really derivative races. There are
indications that such races may have originated in one or more of the
following ways: (_a_) By a natural tendency in synthetic types to
become specialized in the direction of one or other of their
constituent elements. In this way such plants as _Arthrostigma_ and
_Psilophyton_ may have assumed new varietal forms. (_b_) By embryonic
retardation or acceleration,[151] whereby certain species may have had
their maturity advanced or postponed, thus giving them various grades
of perfection in reproduction and complexity of structure. The fact
that so many Erian and Carboniferous plants seem to be on the confines
of the groups of Acrogens and Gymnosperms may be supposed favorable
to such exchanges. (_c_) The contraction and breaking up of floras
which occurred in the Middle Erian and Lower Carboniferous may have
been eminently favorable to the production of such varietal forms as
would result from what has been called the 'struggle for existence.'
(_d_) The elevation of a great expanse of new land at the close of the
Middle Erian and the beginning of the Coal period would, by permitting
the extension of series over wide areas and fertile soils, and by
removing the pressure previously existing, be eminently favorable to
the production of new, and especially of improved, varieties.

"(2.) Whatever importance we may attach to the above supposed causes
of change, we still require to account for the origin of our specific
types. T