I have quoted enough to show that Dr. Erasmus Darwin would have protested against the supposition that functionally produced modifications were an adequate explanation of all the phenomena of organic modification. He declares accident and the chances and changes of this mortal life to be potent and frequent causes of variations, which, being not infrequently inherited, result in the formation of varieties and even species, but considers these causes if taken alone as no less insufficient to account for observable facts than the theory of functionally produced modifications would be if not supplemented by inheritance of so-called fortuitous, or spontaneous variations. The difference between Dr. Erasmus Darwin and Mr. Spencer does not consist in the denial by the first, that a variety which happens, no matter how accidentally, to have varied in a way that enables it to comply more fully and readily with the conditions of its existence, is likely to live longer and leave more offspring than one less favoured; nor in the denial by the second of the inheritance and accumulation of functionally produced modifications; but in the amount of stress which they respectively lay on the relative importance of the two great factors of organic evolution, the existence of which they are alike ready to admit. With Erasmus Darwin there is indeed luck, and luck has had a great deal to do with organic modification, but no amount of luck would have done unless cunning had known how to take advantage of it; whereas if cunning be given, a very little luck at a time will accumulate in the course of ages and become a mighty heap. Cunning, therefore, is the factor on which, having regard to the usage of language and the necessity for simplifying facts, he thinks it most proper to insist. Surely this is as near as may be the opinion which common consent ascribes to Mr. Spencer himself. It is certainly the one which, in supporting Erasmus Darwin’s system as against his grandson’s, I have always intended to support. With Charles Darwin, on the other hand, there is indeed cunning, effort, and consequent use and disuse; nor does he deny that these have produced some, and sometimes even an important, effect in modifying species, but he assigns by far the most important _rôle_ in the whole scheme to natural selection, which, as I have already shown, must, with him, be regarded as a synonym for luck pure and simple. This, for reasons well shown by Mr. Spencer in the articles under consideration, is so untenable that it seems only possible to account for its having been advanced at all by supposing Mr. Darwin’s judgment to have been perverted by some one or more of the many causes that might tend to warp them. What the chief of those causes may have been I shall presently point out.