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