In order to explain how I reached this result, and what it is based upon, it is indispensable that we should first make ourselves acquainted with the phenomena of heredity in general, and the inseparably associated phenomena of reproduction, so that we may form some sort of theoretic conception of the process of inheritance--a picture, necessarily provisional and imperfect, of the mechanism which enables the germ-cell to reproduce the whole organism, and not merely, like other cells, others like itself. We are thus led to an investigation of reproduction and heredity, at the conclusion of which we shall feel justified in returning to the question of the inheritance of acquired characters, in order to give a verdict as to the retention or dismissal of the Lamarckian principle. LECTURE XIII REPRODUCTION IN UNICELLULAR ORGANISMS Reproduction by division--In Amœbæ--In Infusorians--Divisions following one another in immediate succession--Formation of germ-cells in the Metazoa--Contrast between germ-cells and body-cells--Potential immortality of unicellular organisms--Beginning of natural death--Budding and division in the Metazoa. WE wish to consider the reproduction of organisms with special reference to the problem of heredity, and it is most instructive to begin with the lowest forms of life--the unicellulars--because their structure, as far as we can see with the instruments at our command, is very simple, and, what is even more important, is relatively homogeneous. [Illustration: FIG. 59. An Amœba: the process of division. _A_, before the beginning of the division. _B_, the nucleus divided into two. _C_, the two daughter-Amœbæ. Magnified about 400 times.] Suppose that there are bacteria-like organisms of quite homogeneous structure, and that these multiply by simply dividing into two, each rod-like creature dividing transversely in the middle of its length, the two halves would represent independent daughter-organisms, whose structure would correspond exactly with that of the mother-organism, could not indeed in any way deviate from it, and consequently would take over all its characters, that is, would inherit them. The size of body is the only feature which is not obviously inherited, but in reality it is potentially heritable, since the structure of the divided portions involves the capacity and the limits of their possible growth. Moreover, the size of body is not invariable in any species; a particular size is only reproduced under similar conditions of development. Inheritance here consists simply in a continuation of the mother-organism into its two daughter-cells.