s it appears by
Nature."

If we consider, in fact, that by far the largest proportion of recorded
existing species are known only by the study of their skins, or bones,
or other lifeless exuvia; that we are acquainted with none, or next to
none, of their physiological peculiarities, beyond those which can be
deduced from their structure, or are open to cursory observation; and
that we cannot hope to learn more of any of those extinct forms of life
which now constitute no inconsiderable proportion of the known Flora and
Fauna of the world: it is obvious that the definitions of these species
can be only of a purely structural, or morphological, character. It is
probable that naturalists would have avoided much confusion of ideas
if they had more frequently borne the necessary limitations of our
knowledge in mind. But while it may safely be admitted that we are
acquainted with only the morphological characters of the vast majority
of species--the functional or physiological, peculiarities of a few have
been carefully investigated, and the result of that study forms a large
and most interesting portion of the physiology of reproduction.

The student of Nature wonders the more and is astonished the less, the
more conversant he becomes with her operations; but of all the perennial
miracles she offers to his inspection, perhaps the most worthy of
admiration is the development of a plant or of an animal from its
embryo. Examine the recently laid egg of some common animal, such as a
salamander or newt. It is a minute spheroid in which the best microscope
will reveal nothing but a structureless sac, enclosing a glairy fluid,
holding granules in suspension. But strange possibilities lie dormant
in that semi-fluid globule. Let a moderate supply of warmth reach its
watery cradle, and the plastic matter undergoes changes so rapid, yet
so steady and purposelike in their succession, that one can only compare
them to those operated by a skilled modeller upon a formless lump of
clay. As with an invisible trowel, the mass is divided and subdivided
into smaller and smaller portions, until it is reduced to an aggregation
of granules not too large to build withal the finest fabrics of the
nascent organism. And, then, it is as if a delicate finger traced out
the line to be occupied by the spinal column, and moulded the contour
of the body; pinching up the head at one end, the tail at the other,
and fashioning flank and limb into due salamand