"In such a case we have only to assume that the climate which is unfavourable, and the nutriment which is insufficient for horses, affect not only the animal as a whole but also its germ-cells. This would result in the diminution in size of the germ-cells, the effects upon the offspring being still further intensified by the insufficient nourishment supplied during growth. But such results would not depend upon the transmission by the germ-cells of certain peculiarities due to the unfavourable climate, which only appear in the full-grown horse." But Professor Weismann does not like such cases, and admits that he cannot explain the facts in connection with the climatic varieties of certain butterflies, except "by supposing the passive acquisition of characters produced by the direct influence of climate." Nevertheless in his next paragraph but one he calls such cases "doubtful," and proposes that for the moment they should be left aside. He accordingly leaves them, but I have not yet found what other moment he considered auspicious for returning to them. He tells us that "new experiments will be necessary, and that he has himself already begun to undertake them." Perhaps he will give us the results of these experiments in some future book--for that they will prove satisfactory to him can hardly, I think, be doubted. He writes:-- "Leaving on one side, for the moment, these doubtful and insufficiently investigated cases, we may still maintain that the assumption that changes induced by external conditions in the organism as a whole are communicated to the germ-cells after the manner indicated in Darwin's hypothesis of pangenesis, is wholly unnecessary for the explanation of these phenomena. Still we cannot exclude the possibility of such a transmission occasionally occurring, for even if the greater part of the effects must be attributable to natural selection, there might be a smaller part in certain cases which depends on this exceptional factor."