|
THE PERPETUATION OF LIVING BEINGS
THE PERPETUATION OF LIVING BEINGS THOMAS H. HUXLEY by Thomas Henry Huxley
The inquiry which we undertook at our last meeting into the state of
our knowledge of the causes of the phenomena of organic nature--of the
past and of the present--resolved itself into two subsidiary
inquiries: the first was whether we know anything either historically
or experimentally of the mode of origin of living beings; the second
subsidiary inquiry was whether granting the origin we know anything
about the perpetuation and modifications of the forms of organic
beings. The reply which I had to give to the first question was
altogether negative and the chief result of my last lecture was that
neither historically nor experimentally do we at present know anything
whatsoever about the origin of living forms. We saw that historically
we are not likely to know anything about it although we may perhaps
learn something experimentally; but that at present we are an enormous
distance from the goal I indicated.
I now then take up the next question What do we know of the
reproduction the perpetuation and the modifications of the forms of
living beings supposing that we have put the question as to their
origination on one side and have assumed that at present the causes of
their origination are beyond us and that we know nothing about them?
Upon this question the state of our knowledge is extremely different;
it is exceedingly large and if not complete our experience is
certainly most extensive. It would be impossible to lay it all before
you and the most I can do or need do to-night is to take up the
principal points and put them before you with such prominence as may
subserve the purposes of our present argument.
The method of the perpetuation of organic beings is of two kinds--the
asexual and the sexual. In the first the perpetuation takes place from
and by a particular act of an individual organism which sometimes may
not be classed as belonging to any sex at all. In the second case it
is in consequence of the mutual action and interaction of certain
portions of the organisms of usually two distinct individuals--the
male and the female. The cases of asexual perpetuation are by no means
so common as the cases of sexual perpetuation; and they are by no means
so common in the animal as in the vegetable world. You are all
probably familiar with the fact as a matter of experience that you
can propagate plants by means of what are called "cuttings;" for
example that by taking a cutting from a geranium plant and rearing it
properly by supplying it with light and warmth and nourishment from
the earth it grows up and takes the form of its parent having all the
properties and peculiarities of the original plant.
Sometimes this process which the gardener performs artificially takes
place naturally; that is to say a little bulb or portion of the
plant detaches itself drops off and becomes capable of growing as a
separate thing. That is the case with many bulbous plants which throw
off in this way secondary bulbs which are lodged in the ground and
become developed into plants. This is an asexual process and from it
results the repetition or reproduction of the form of the original
being from which the bulb proceeds.
Among animals the same thing takes place. Among the lower forms of
animal life the infusorial animalculae we have already spoken of throw
off certain portions or break themselves up in various directions
sometimes transversely or sometimes longitudinally; or they may give
off buds which detach themselves and develop into their proper forms.
There is the common fresh-water Polype for instance which multiplies
itself in this way. Just in the same way as the gardener is able to
multiply and reproduce the peculiarities and characters of particular
plants by means of cuttings so can the physiological
experimentalist--as was shown by the Abbe Trembley many years ago--so
can he do the same thing with many of the lower forms of animal life.
M. de Trembley showed that you could take a polype and cut it into two
or four or many pieces mutilating it in all directions and the pieces
would still grow up and reproduce completely the original form of the
animal. These are all cases of asexual multiplication and there are
other instances and still more extraordinary ones in which this
process takes place naturally in a more hidden a more recondite kind
of way. You are all of you familiar with those little green insects
the 'Aphis' or blight as it is called. These little animals during a
very considerable part of their existence multiply themselves by means
of a kind of internal budding the buds being developed into
essentially asexual animals which are neither male nor female; they
become converted into young 'Aphides' which repeat the process and
their offspring after them and so on again; you may go on for nine or
ten or even twenty or more successions; and there is no very good
reason to say how soon it might terminate or how long it might not go
on if the proper conditions of warmth and nourishment were kept up.
Sexual reproduction is quite a distinct matter. Here in all these
cases what is required is the detachment of two portions of the
parental organisms which portions we know as the egg and the
spermatozoon. In plants it is the ovule and the pollen-grain as in the
flowering plants or the ovule and the antherozooid as in the
flowerless. Among all forms of animal life the spermatozoa proceed
from the male sex and the egg is the product of the female. Now what
is remarkable about this mode of reproduction is this that the egg by
itself or the spermatozoa by themselves are unable to assume the
parental form; but if they be brought into contact with one another
the effect of the mixture of organic substances proceeding from two
sources appears to confer an altogether new vigour to the mixed product.
This process is brought about as we all know by the sexual
intercourse of the two sexes and is called the act of impregnation.
The result of this act on the part of the male and female is that the
formation of a new being is set up in the ovule or egg; this ovule or
egg soon begins to be divided and subdivided and to be fashioned into
various complex organisms and eventually to develop into the form of
one of its parents as I explained in the first lecture. These are the
processes by which the perpetuation of organic beings is secured. Why
there should be the two modes--why this re-invigoration should be
required on the part of the female element we do not know; but it is
most assuredly 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
...
|
