The long communication above quoted led to a still longer correspondence in the pages of _Nature_. For Professor Ray Lankester wrote[141] to impugn the doctrine of panmixia, or cessation of selection, _in toto_, arguing with much insistence that "cessation of selection must be supplemented by economy of growth in order to produce the results attributed to panmixia." In other words, he denied that panmixia alone can cause degeneration in any degree at all; at most, he said, it can be but "a condition," or "a state," which occurs when an organ or part ceases to be useful, and therefore falls under the degenerating influence of active causes, such as economy of nutrition. Or, in yet other words, he refused to recognize that any degenerative process can be due to natural selection as merely withdrawn: only when, besides being _withdrawn_, natural selection is _reversed_, did he regard a degenerative process as possible. As a result of the correspondence, however, he eventually[142] agreed that, if the "birth-mean" of an organ, in respect either of size or complexity of structure, be lower than the "selection-mean" while the organ is useful (a fact which he does not dispute); then, if the organ ceases to be useful, it will degenerate by the withdrawal of selection alone. Which, of course, is merely a re-statement of the doctrine of panmixia, or cessation of selection, in somewhat varied terminology--provided that the birth-mean be taken over a number of generations, or not only over a few following the selection-mean of the structure while still in its highest state of efficiency. For the sake of brevity I will hereafter speak of these "few following" generations by the term of "first generations." [141] _Nature_, vol. xli. p. 486. [142] _Ibid._ vol. xlii. p. 52. It remains to consider the views of Professor Lloyd Morgan upon the subject. In my opinion he is the shrewdest, as well as the most logical critic that we have in the field of Darwinian speculation; therefore, if possible, I should like to arrive at a full agreement with him upon this matter. His latest utterance with regard to it is as follows:--