Postscript.

This paper has been submitted to Professor Hugo de Vries, and he has sent me this comment:—

“It is, of course, interesting for me to read a statement of my views from a neo-Lamarckian standpoint, and the concession that the facts described by you do not contain any argument for a decision between the two contrasting theories.

“For me your article shows that R. paucifolius, R. chordorhizos, R. crithmifolius, and R. Haastii must have had a common ancestor, which was already a xerophyte, and that they must have inherited this character from it. This ancestor may have had the same geographical distribution which is now shown by the aggregate of its descendants. Perhaps one of them is identical with it; perhaps it has wholly disappeared. Under what conditions it lived we can, of course, not know, nor where and when it acquired its xerophytic properties. To conclude that it must have acquired them in a period of drought would be a circulus vitiosus, since it would simply be applying the theory to a special case and then considering the case as a proof of the theory.

“You say that possibly new characters may be acquired by a plant as a direct response to Nature's ultimatum, ‘Change or die.’ This is the old view, but not mine. The article you quote from was just intended to show that, as far as we know, the response has, as a matter of fact, always been, ‘I cannot change at your will and so I must die.’

“You assume that your plants have passed through periods of moisture, but have retained their xerophytic character nevertheless. It seems to me that this is conceding that external conditions do not, as a rule, provoke corresponding useful changes. They may do so, or seem to do so, or they may not. My view, that mutations, although, of course, caused by external conditions, are not necessarily responses to the ‘demands of a new stress,’ seems quite adequate to interpret your facts. I gladly concede that the causes of mutations are still dark to us, but then I say that responses such as Warming and other neo-Lamarckians suppose are far darker. Especially if you take into consideration what is now known concerning the structure of chromosomes and the distribution of the hereditary characters in them, it seems impossible to imagine the nature of such a supposed response. On the other hand, if we do not know the causes of mutation, the fact of their occurrence has been proved in so numerous individual cases that it can no longer be doubted, even by those who want to exclude the Oenotheras from the discussion.

“I shall be very glad to learn the results of your garden cultures. I should not wonder if your plants would behave just like the creosotbush of Tucson, and prefer better conditions to those which they enjoy (?) just-now. To me it seems that plants are found in those localities where they can better endure the circumstances than their competitors. But whether they really enjoy them, or would prefer more moisture and more fertile soils, and so on, is another question.”