It is necessary to point this out, because the difference between the almost universal rejection of Lamarck's attempted solution of the problem of evolution, and the almost immediate and universal acceptance of that adduced by Darwin, is otherwise unexplained. The belief in the doctrine of evolution as the only rational explanation of the gradual development of the innumerable forms of living things became more and more general. The great body of arguments in its favour were admirably set forth by Robert Chambers in his _Vestiges of Creation_, published anonymously in 1844; while Herbert Spencer's masterly exposition of the argument for universal evolution convinced a large number of naturalists and men of science. But still the nature of the laws and forces by which the evolution of the organic world in all its variety and beauty, could have been brought about remained not only unknown but unimagined, so that even so great a thinker as Sir John Herschel termed it "the mystery of mysteries." I will now state as briefly as possible the essential features of Darwin's solution of the mystery in his epoch-making work, _The Origin of Species_. _Natural Selection as the Essential Factor in the Origin of Species_ There are two great, universal, and very conspicuous characteristics of the whole organic world which, because they are so very common, were almost ignored before Darwin showed their importance. These are (1) the great _variability_ in all common and widespread species, and (2) their enormous powers of _increase_.