Such certainly do exist.

They are fertile together, and yet each maintains itself tolerably distinct.
How can this be, if there is no disinclination to crossing?
My belief certainly is that number of offspring is not so important an element in keeping up population of a species as supply of food and other favourable conditions; because the numbers of a species constantly vary greatly in different parts of its own area, whereas the average number of offspring is not a very variable element.
However, I will say no more, but leave the problem as insoluble, only fearing that it will become a formidable weapon in the hands of the enemies of Natural Selection.
LETTER 215.

TO J.D.

HOOKER.
(215/1.

The following extract from a letter to Sir Joseph Hooker (dated April 3rd, 1868) refers to his Presidential Address for the approaching meeting of the British Association at Norwich.
Some account of Sir Joseph's success is given in the "Life and Letters," III., page 100, also in Huxley's "Life," Volume I., page 297, where Huxley writes to Darwin:-- "We had a capital meeting at Norwich, and dear old Hooker came out in great force, as he always does in emergencies.