I think it would be better to do away with all such qualifying expressions, and constantly maintain (what I certainly believe to be the fact) that variations of every kind are always occurring in every part of every species, and therefore that favourable variations are always ready when wanted.

You have, I am sure, abundant materials to prove this; and it is, I believe, the grand fact that renders modification and adaptation to conditions almost always possible.

I would put the burthen of proof on my opponents to show that any one organ, structure, or faculty does not vary, even during one generation, among all the individuals of a species; and also to show any mode or way in which any such organ, etc., does not vary.

I would ask them to give any reason for supposing that any organ, etc., is ever absolutely identical at any one time in all the individuals of a species, and if not then it is always varying, and there are always materials which, from the simple fact that "the fittest survive," will tend to the modification of the race into harmony with changed conditions.
I hope these remarks may be intelligible to you, and that you will be so kind as to let me know what you think of them.
I have not heard for some time how you are getting on.

I hope you are still improving in health, and that you will now be able to get on with your great work, for which so many thousands are looking with interest.
LETTER 191.