Then we may note that very low in the scale of living things there is a process of reproduction known as conjugation, in which, although the cells of the species appear to be all alike, yet, nevertheless, two of them join together for purposes of reproduction. In other words we have here a process of cell-union before we have the cell-division which follows. It is important to note at this stage that the creatures which we have mentioned, and even some more highly organised, such as an am[oe]ba, which has a nucleus, go through these simple or complicated reproductive processes in the total absence of anything which could suggest a distinction of sex. In these cases the individuals are obviously all of one sex, and, therefore, the distinction of sexes into male and female is evidently something which has been added later in the scheme of evolution, not for the purpose of reproduction itself, but for something which is to be added to that. Then in the slightly higher animals and plants we come to those in which many cells go to the making of the individual, the multicellular individuals, and amongst these we very soon see the origin of what is termed specialisation of function. That is to say, in these higher creatures which consist of many numbers of cells arranged so as to form one individual, certain cells are set apart for one purpose and others for another. Some may be for digestion, some for purposes of movement, and others for reproduction. Here we have a new phenomenon, namely, the setting aside of certain cells in a multicellular individual which from the very beginning are capable of one function alone, namely, reproducing the species. The higher one goes in the scale of life the more striking and obvious this fact becomes, and as we shall see when we come to the vertebrate kingdom, this setting aside of the cells which are to produce the individuals of the next generation is the key to the solution of the most difficult of our problems.