By reading pages 401 and 404 of my paper, you will see why such cases are of quite as much importance to me as the converse, viz. where closely allied species inhabiting _continuous_ areas are more or less mutually sterile (see p. 392). If you have hitherto failed to apply these converse tests to my theory, I cannot conceive by what other principle you have sought to test it. Pray read the passages referred to, which present the shortest summary of what I regard as the very backbone of my evidence. If your large knowledge of geographical distribution should enable you to supply me with specific cases of the general principle mentioned by Darwin in the quotation given on page 392 ('Origin of Species,' 6th ed., pp. 134-5), I should much like to try experiments on the sterility which I should expect to find between these interlocking species. It seems comical to ask a scientific opponent for assistance, but the fact of being able to do so proves the superiority of science to politics. December 25, 1888. It is very good of you to write such a long and suggestive letter. As a result of attentively reading your letter, it appears to me that you think I suppose sterility in a high degree to be much more usual among allied species than I do suppose it. I well know the large amount of natural as well as artificial hybridisation that goes on. But, on the other hand, there are so many species which either will not cross at all, or produce sterile hybrids, that, taking a general view of all species together, mutual sterility does become by far the most generally distributed single peculiarity--_i.e._ is the one peculiarity which, more than any other that can be named, is common to numberless species. Thus much for mutual sterility that is absolute, either in first crosses or in their hybrid progeny. But now, the most important thing for me is mutual sterility that is _not_ absolute (though, on my theory, perhaps on its way to becoming so) but relative, _i.e._ there being a _lower degree of fertility_ between A × B or B × A, than there is between A × A or B × B.