Where then are we?
Clearly we have not really produced any new species in any correct sense
of the word. If we have produced new forms that breed true and that are
seemingly just as deserving of the rank of distinct species as many now
listed in scientific books, it only shows that our lists are sadly at
fault, and that they are not all species that are called species. These
experiments merely indicate that _the parent form possesses more
potential characters than it can give expression to in a single
individual form_, some of them being necessarily latent or hidden, and
that when these latent ones show themselves they must do so at the
expense of others which become latent or hidden in their turn. This
_vital elasticity_, as it may be termed, or the vital rebound under
definite conditions, is indeed a prime characteristic of the species
just as it is of the individual; but like that of the individual the
vital elasticity of the species is strictly bounded by comparatively
narrow limits beyond which we have never seen a single type pass under
either natural or artificial conditions. Mutations can be made according
to Mendel's Law; but when we have made them once _we can always be sure
of producing the_ _very same mutants again in the very same way_, as
surely as we produce a definite chemical compound; and when we have made
it _we can always resolve it at will back into its original form_, just
as we can a chemical compound. And so, where is the evolution? or how do
these facts throw any light on the problem of the origin of species, any
more than chemical compounds throw light on the origin of the elements?
Obviously in biology as in chemistry we are only working in a circle,
merely marking time.
And the bearing of these facts on the other problem of the transmission
of acquired characters is quite obvious. Mendelism provides no place for
any such transmission. Mendel's Law is sometimes called the law of
_alternative inheritance_, thus embodying in its name the thought that
offspring may show the characters possessed by one parent or by the
other, but that it cannot develop any characters whatever which were not
manifest or latent in the ancestry. Changes in the environment during
the embryonic stage, it is true, seem sometimes to be registered in the
growing form; but it has never yet been proved that these induced
changes can ever amount to a unit character or genetic factor that will
maintain itself and segregate as a distinct factor after hybridization.
Ancestry alone furnishes the material for the factor, and no amount of
induced change can get itself registered in the organism so as to come
into this charmed circle of ancestral characters which alone seem to be
passed on to posterity.
A quotation from Bateson ought to set this point at rest:
"The essence of the Mendelian principle is very easily expressed. It is,
first, that in great measure the properties of organisms are due to the
presence of distinct, detachable elements [factors], separately
transmitted in heredity; and secondly, that _the parent cannot pass on
to offspring an element, and consequently the corresponding property,
which it does not itself possess_."[28]
[Footnote 28: _Scientific American_ Sup., January 3, 1914.]
Heredity we now see is a method of analysis, and the facts brought to
light by Mendelism help us very much toward an understanding of living
matter. Especially does it help us to understand the complexity
underlying the facts of heredity, which until now have seemed so strange
and capricious. As Professor Punnett of Cambridge remarks:
"Constitutional differences of a radical nature may be concealed beneath
an apparent identity of external form. Purple sweet peas from the same
pod, indistinguishable in appearance and of identical ancestry, may yet
be fundamentally different in their constitution. From one may come
purples, reds, and whites; from another only purples and reds; from
another purples and whites alone; whilst a fourth will breed true to
purple. Any method of investigation which fails to take account of the
radical differences of constitution which may underlie external
similarity, must necessarily be doomed to failure. Conversely, we
realize to-day that individuals identical in constitution may yet have
an entirely different ancestral history. From the cross between two
fowls with rose and pea combs, each of irreproachable pedigree for
generations, come single combs in the second generation, _and these
singles are precisely similar in their behavior to singles bred from
strains of unblemished ancestry_. In the ancestry of the one is to be
found no single over a long series of years; in the ancestry of the
other nothing but singles occurred. The creature of given constitution
may often be built up in many ways, but once formed it will behave like
others of the same constitution."[29]
[Footnote 29: Encyclopædia Britannica, Vol. XVIII, p. 119.]