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