The Perpetuation of Living Beings, Hereditary Transmission and Variation | Page 5

the fact, and it is
presumable, that, however long the process of asexual multiplication
could be continued, I say there is good reason to believe that it would
come to an end if a new commencement were not obtained by a
conjunction of the two sexual elements.
That character which is common to these two distinct processes is this,
that, whether we consider the reproduction, or perpetuation, or
modification of organic beings as they take place asexually, or as they
may take place sexually,--in either case, I say, the offspring has a
constant tendency to assume, speaking generally, the character of the
parent. As I said just now, if you take a slip of a plant, and tend it with
care, it will eventually grow up and develop into a plant like that from
which it had sprung; and this tendency is so strong that, as gardeners
know, this mode of multiplying by means of cuttings is the only secure
mode of propagating very many varieties of plants; the peculiarity of
the primitive stock seems to be better preserved if you propagate it by
means of a slip than if you resort to the sexual mode.
Again, in experiments upon the lower animals, such as the polype, to
which I have referred, it is most extraordinary that, although cut up into
various pieces, each particular piece will grow up into the form of the
primitive stock; the head, if separated, will reproduce the body and the
tail; and if you cut off the tail, you will find that that will reproduce the
body and all the rest of the members, without in any way deviating
from the plan of the organism from which these portions have been
detached. And so far does this go, that some experimentalists have
carefully examined the lower orders of animals,--among them the Abbe
Spallanzani, who made a number of experiments upon snails and
salamanders,--and have found that they might mutilate them to an
incredible extent; that you might cut off the jaw or the greater part of
the head, or the leg or the tail, and repeat the experiment several times,
perhaps, cutting off the same member again and again; and yet each of
those types would be reproduced according to the primitive type: nature
making no mistake, never putting on a fresh kind of leg, or head, or tail,
but always tending to repeat and to return to the primitive type.

It is the same in sexual reproduction: it is a matter of perfectly common
experience, that the tendency on the part of the offspring always is,
speaking broadly, to reproduce the form of the parents. The proverb has
it that the thistle does not bring forth grapes; so, among ourselves, there
is always a likeness, more or less marked and distinct, between children
and their parents. That is a matter of familiar and ordinary observation.
We notice the same thing occurring in the cases of the domestic
animals--dogs, for instance, and their offspring. In all these cases of
propagation and perpetuation, there seems to be a tendency in the
offspring to take the characters of the parental organisms. To that
tendency a special name is given-- it is called 'Atavism', it expresses
this tendency to revert to the ancestral type, and comes from the Latin
word 'atavus', ancestor.
Well, this 'Atavism' which I shall speak of, is, as I said before, one of
the most marked and striking tendencies of organic beings; but, side by
side with this hereditary tendency there is an equally distinct and
remarkable tendency to variation. The tendency to reproduce the
original stock has, as it were, its limits, and side by side with it there is
a tendency to vary in certain directions, as if there were two opposing
powers working upon the organic being, one tending to take it in a
straight line, and the other tending to make it diverge from that straight
line, first to one side and then to the other.
So that you see these two tendencies need not precisely contradict one
another, as the ultimate result may not always be very remote from
what would have been the case if the line had been quite straight.
This tendency to variation is less marked in that mode of propagation
which takes place asexually; it is in that mode that the minor characters
of animal and vegetable structures are most completely preserved. Still,
it will happen sometimes, that the gardener, when he has planted a
cutting of some favourite plant, will find, contrary to his expectation,
that the slip grows up a little different from the primitive stock--that it
produces flowers of a different colour or make, or some deviation in
one way or another. This is what is called the 'sporting' of plants.
In