“On critically examining the evidence” (modern writers never examine evidence, they always “critically,” or “carefully,” or “patiently,” examine it), he writes, “we shall find reason to think that it by no means explains all that has to be explained. Omitting for the present any consideration of a factor which may be considered primordial, it may be contended that one of the factors alleged by Erasmus Darwin and Lamarck must be recognised as a co-operator. Unless that increase of a part resulting from extra activity, and that decrease of it resulting from inactivity, are transmissible to descendants, we are without a key to many phenomena of organic evolution. _Utterly inadequate to explain the major part of the facts as is the hypothesis of the inheritance of functionally produced modifications_, yet there is a minor part of the facts very extensive though less, which must be ascribed to this cause.” (Italics mine.) Mr. Spencer does not here say expressly that Erasmus Darwin and Lamarck considered inheritance of functionally produced modifications to be the sole explanation of the facts of organic life; modern writers on evolution for the most part avoid saying anything expressly; this nevertheless is the conclusion which the reader naturally draws—and was doubtless intended to draw—from Mr. Spencer’s words. He gathers that these writers put forward an “utterly inadequate” theory, which cannot for a moment be entertained in the form in which they left it, but which, nevertheless, contains contributions to the formation of a just opinion which of late years have been too much neglected. This inference would be, as Mr. Spencer ought to know, a mistaken one. Erasmus Darwin, who was the first to depend mainly on functionally produced modifications, attributes, if not as much importance to variations induced either by what we must call chance, or by causes having no connection with use and disuse, as Mr. Spencer does, still so nearly as much that there is little to choose between them. Mr. Spencer’s words show that he attributes, if not half, still not far off half the modification that has actually been produced, to use and disuse. Erasmus Darwin does not say whether he considers use and disuse to have brought about more than half or less than half; he only says that animal and vegetable modification is “in part produced” by the exertions of the animals and vegetables themselves; the impression I have derived is, that just as Mr. Spencer considers rather less than half to be due to use and disuse, so Erasmus Darwin considers decidedly more than half—so much more, in fact, than half as to make function unquestionably the factor most proper to be insisted on if only one can be given. Further than this he did not go. I will quote enough of Dr. Erasmus Darwin’s own words to put his position beyond doubt. He writes:—