The question, then, that we have to put to any theory of evolution--that is, to any theory which professes to state the process by which the totality of things has come to be what it is--is, "Does it account for that totality? do the causes which it assumes to have been at work account for all that is?" Now, _a priori_ it was not to be expected that evolution would in its infancy, and it is still young, succeed in accounting for all things; and there were special reasons in the circumstances under which it first took its modern scientific shape which necessarily limited its earliest attempts to grasp the totality of things. It would, however, be absurd to judge the principle by the first attempt to apply it, and to condemn it because it has not done in a moment what with time it assuredly will succeed in effecting. At the same time, it can only effect that wider success by refusing to stereotype its first errors and by declining to bind itself to the dogma that what it has succeeded in explaining is all that there is to explain, or that that alone is or happens which its present assumptions or laws are capable of accounting for. There lies the danger which threatens to check the further development of the theory of evolution--in the dogmatism which pretends to set aside common sense and the common reason, and arrogates to itself the sole right of saying what is; and succeeds in doing so by the simple but circular argument that that alone is or happens which can be accounted for by the laws that regulate the movements of things in space or that follow from the struggle for animal existence.