To J. Harris   21 May 1880

Dear Sir

No one could answer your question.¹ I do not believe one organism can be named, of which we know precisely the conditions of its existence. We cannot explain why one species is common and another rare in the same district, or why one is wholly absent from another district; we can give no explanation why certain species have increased in number during the last $\frac{1}{2}$ century in this country. Until we can explain such comparatively simple cases, it is not likely that we should be able to say why one form has been modified and not another; or why one species has become extinct and not another.

Dear Sir | Yours faithfully | Ch. Darwin