In general, the theory of evolution is an assertion of the unity of life, or, in its widest form, of the unity of existence. Progressive modifications and hereditary transmission of such modifications are, it is contended, sufficient to explain the different forms and species which life now manifests. The assumption is specially discarded that there are fixed differences between kinds of living things making it impossible for them all to have developed from simple germs, originally of like constitution, which have, in the course of time, become more heterogeneous and complex, and so given rise to the wealth of organic life. But this general doctrine, held (wholly or in part) in modern times by Kant, Wolff, and Lamarck, needed to be supplemented by a definite view of the way in which the progressive modifications took place; and this required to be established as a really operative cause, before evolution could receive scientific proof. This more special element of the theory was Darwin's contribution to the subject. Evolution, he showed,--and herein consists his theoretical advance on Lamarck,--has taken place by the "natural selection" of organisms, so modified as to fit them for survival in the struggle for existence. Organisms in which advantageous modifications have been produced tend to survive, and to transmit their modified structure to descendants, while organisms in which such modifications have not been produced, are less able to preserve their life and to hand it on to successors. Older types, it is true, remain, but only in circumstances in which their continued existence does not seriously interfere with the organisms which, in the struggle for life, have developed a structure better suited to their environment: when more perfect and less perfect forms cannot exist together, only the better adapted survive. [Sidenote: in first instance, historical;] [Sidenote: but implies a teleological aspect, which may have ethical consequences.]