apprehend," says Professor Owen,[61] "that few naturalists
now-a-days, in describing and proposing a name for what they call
'a new _species_,' use that term to signify what was meant by it
twenty or thirty years ago; that is, an originally distinct
creation, maintaining its primitive distinction by obstructive
generative peculiarities. The proposer of the new species now
intends to state no more than he actually knows; as, for example,
that the differences on which he founds the specific character are
constant in individuals of both sexes, so far as observation has
reached; and that they are not due to domestication or to
artificially superinduced external circumstances, or to any outward
influence within his cognizance; that the species is wild, or is
such as 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 these 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 a newt. It is a minute spheroid in which the best
microscope will reveal nothing but a structureless sac, enclosing a