From this position he draws as a corollary, that the wider the limits of selection, the keener the rivalry and the more rigid the selection, just so much greater will be the progress; while for any progress at all there must be some rivalry in selection, so that every progressive form must lead a life of continual strain and stress as it travels its upward path. This again is true in a measure, but is not true as broadly as Mr. Kidd has stated it. The rivalry of natural selection is but one of the features in progress. Other things being equal, the species where this rivalry is keenest will make most progress; but then “other things” never are equal. In actual life those species make most progress which are farthest removed from the point where the limits of selection are very wide, the selection itself very rigid, and the rivalry very keen. Of course the selection is most rigid where the fecundity of the animal is greatest; but it is precisely the forms which have most fecundity that have made least progress. Some time in the remote past the guinea pig and the dog had a common ancestor. The fecundity of the guinea pig is much greater than that of the dog. Of a given number of guinea pigs born, a much smaller proportion are able to survive in the keen rivalry, so that the limits of selection are wider, and the selection itself more rigid; nevertheless the progress made by the progenitors of the dog since eocene days has been much more marked and rapid than the progress made by the progenitors of the guinea pig in the same time.