Dear Sir,

I thank you for your very kind, long & interesting letter¹   The case is so wonderful & difficult that I dare not express any opinion on it. Of course I regret that Hilgendorf has been proved to be so greatly in error, but it is some selfish comfort to me that I always felt so much misgiving that I never quoted his paper.² The variability of these shells is quite astonishing, & seems to exceed that of Rubus or Hieracium amongst plants.³ The result which surprises me most is that the same form should be developed from various & different progenitors. This seems to show how potent are the conditions of life, irrespectively of the variations being in any way beneficial. The production of a species out of a chaos of varying forms reminds me of Nägelis conclusion as deduced from the study of Hieracium that this is the common mode in which species arise.⁴ But I still continue to doubt much on this head, & cling to the belief expressed in the first edit of the Origin that protean or Polymorphic species are those which are now varying in such a manner that the variations are neither advantageous nor disadvantageous.⁵ I am glad to hear of the Brunswick deposit, as I feel sure that the careful study of such cases is highly important.⁶

I hope that the Smithsonian Institⁿ. will publish your memoir,⁷ & I remain | dear Sir | Yours faithfully | Ch. Darwin