The lowest form of the reproductive process is, of course, by simple division and redivision. This is characteristic of many of those organisms which consist only of a single cell, and it may co-exist, even in these, with a considerable degree of structural complexity, as in the ‘trumpet animalcule,’ _Stentor raselii_. But among the lowest of these unicellular organisms a curious process is sometimes observed to take place, in which we may doubtless recognize the origin of sexual reproduction. Two, three, or more _Amœbæ_[42] approach each other, partially coalesce, and remain united for some time. They then separate again. No new creatures are formed by this contact; there are no visible results at all. But that _something_ which is for the advantage of the organisms takes place during this period of union is certain, and in the light of what is known of processes in other organisms we can make a very good guess at what this something is. Each _Amœba_ parts with some of its chromatin to some other and receives an equivalent in exchange. The creature is thus reconstituted. The element of _change_, which always provides so marked a stimulus to vital processes, has been obtained. The process has actually been observed in a certain Infusorian, _Noctiluca_. Two _Noctilucas_ coalesce, and then proceed to divide _at right angles to the plane of contact_. This necessarily has the effect of giving to each of the two new _Noctilucas_ which result from the division half the nucleus and chromatin of one parent and half of the other. There is, however, no actual new birth or multiplication of beings; there are only two _Noctilucas_ as before. We can now imagine that if a certain class of unicellular organisms are in the habit of approaching each other for the purpose of this interchange of portions of their chromatin, they might occasionally, under the influence of the approaching conjugation, expel those portions of chromatin _before_ another cell was in a position to receive it. What would happen if two cells, each of which had thus got rid of half its chromatin, were to come into contact? Plainly, they would fuse together; they would not separate again; they would become a new organism. Each would have supplied just what the other lacked.