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of Virchow, who in his Pathologic cellulaire shows that the cell is not only nourished, but that it nourishes itself by its own activity. Furthermore, Weismann, in accordance with modern embryology, brings out in connection with the segmentation of the egg and the phenomena of consecutive development, the proof that it is in the cells themselves that we find the reasons for the different forms of reproduction.
Therefore the theory of natual selection suffices to explain the formation of species and races. It has for its basis the vari- ability of organisms. This variability brings about great prepa- rations for the changes in their conditions of existence. These new conditions impose upon the organism new exigencies to which they are obliged to yield in order to exist. Then intervenes a progressive and continued selection which so acts among the modified organisms that only those survive which are the better adapted to the modified conditions of existence. Selection con- tinuing in this way, the differences between the primitive forms and the derived forms tend more and more to accentuate them- selves. They become differences of race, and even of species.
Weismann adds:
These modifications in the conditions of existence, as well as in those of the organism, must be effected very slowly and by short steps. At any moment in the whole phenomenon of transformation the species must live insufficiently accommodated to the conditions of existence. A sudden brusque transformation cannot be conceived, for the reason that it would render the existence of the species impossible.
This law of constant and general equilibrium accompanies at each step all of the movements and all of the variations of indi- vidual organisms, and we shall see it operating also among the social organisms. We shall see, in the proper connection, that it is these same laws that we must employ as the scientific bases of the limits in time and space of the development of the social organizations. Thus the theory of frontiers constantly follows a line parallel to that of the theories of the natural sciences in general.
In reality, heredity is not abolished, let us truly remark, but it is explained through the continuity of variations, in the sense
