This is also quoted from the Rev. Prof. Henslow's letters to me, and the important point in it is, that the great changes in question are proved to be of a purely "somatogenetic" kind; for they occurred "at once" _in the ready-grown plant_, when the organs concerned were exposed to the change from aquatic to terrestrial life, or _vice versa_--and also from a subterranean to an aerial position, or _vice versa_. Consequently, even the abstract possibility of the changed conditions of life having operated on the _seed_ is here excluded. Yet the changes are of precisely the same kind as are now _hereditary_ in the wild species. It thus appears undeniable that all these remarkable and uniform changes must originally have been somatogenetic changes; yet they have now become blastogenetic. This much, I say, seems undeniable; and therefore it goes a long way to prove that the non-blastogenetic character of the changes has been due to their originally somatogenetic character. For, if not, how did natural selection ever get an opportunity of making any of them blastogenetic, when every individual plant has always presented them as already given somatogenetically? This last consideration appears in no small measure to justify the opinion of Mr. Henslow, who concludes--"These experiments prove, not only that the influence of the environment is _at once_ felt by the organ; but that it is indubitably the _cause_ of the now specific and hereditary traits peculiar to normally aquatic, subterranean, and aerial stems, or roots[80]." [80] It also serves to show that Weismann's newer doctrine of similar "determinants" occurring both in the germ and in the somatic tissues is a doctrine which cannot be applied to rebut this evidence of the transmission of acquired characters in plants. Therefore even its hypothetical validity as applied by him to explain the seasonal variation of butterflies is rendered in a high degree dubious. He continues to furnish other instances in the same line of proof--such as the distinctive "habits" of insectivorous, parasitic, and climbing plants; the difference in structure between the upper and under sides of horizontal leaves, &c. "For here, as in all organs, we discover by experiment how easily the anatomy of plants can be affected by their environment; and that, as long as the latter is constant, so are the characters of the plants constant and hereditary." [The following letter, contributed by Dr. Hill to _Nature_, vol. I. p. 617, may here be quoted. C. Ll. M.