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Show 86 THE PERPE'rUATION 01<' LIVING llEI~GSJ asexual multiplication, and there are other instances, and still more extraordinary ones, in which this pro-cess takes place naturally, in a more hidden, a n1ore 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 nourishn1ent were kept up. Sexual reproduction is quite a distinct 1natter. I-Iere, in all these cases, what is required is the detachment of two portions of the parental organisms, which portions we know as the egg or the spermatozoon. In plants it is the ovule and the pollen-grain, as in the flowering plants, or the ovule and the antherozooicl,. as in the flowerless. Among all forms of animal life, the spermatozoa proceed from the male sex, and the eo-o- is the product of the female. Now, what is 00 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 pro- HEREDITARY THANS:\HSSION AND VAltiATION. 87 cccding from two sources appears to confer an altogether new vigour to the mixed product. This process is brought about, as 've all know, by the sexual intercourse of the two sexes, and is called the act of impregnation. r:rhe 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. vVhy there sl10uld 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 encl if a new commencement were not obtained by a conjunction of the two sexual elements. Tl;lat character which is common to these two distinct processes is this, that, whether we consider the reproduction, . or perpetuation, or modification of or<ranic beinn·s as thev take place asexually, or as t:> b ~ they may take place sexually,-in either case, I say, the ofi"spring 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 teud it with care, it will eventually grow up and de ·velop into a plant like that from which it had sprung~ .and this tendency is so strong that, as gardeners know, |