Where only one individual out of a thousand or a million can survive to maturity and propagation, clearly only the very best adapted to the environment will so survive; and in somewhat different environments, only those best adapted respectively to the two, three, or more different environments. The intermediates, i. e., those least particularly adapted to the two or three different emergencies, surely have least chance. Now our species of plants and animals are the comparatively few surviving lines of a great number of lines which have come down through long and various periods of great tribulation, in the literal sense of the word; they have been ground over and over, first on this trial, then on that, leaving, as it seems to me, no chance of the survival, side by side, of all sorts and shades of intermediate forms. Hence, under this, and the general law of heredity, the practical distinction of species and genera appears to be a natural result. That low, and even the lowest, forms of life should survive and abound all down the ages, and be the most widely diffused over the earth, seems also the most natural result, being simple adaptations to simplest conditions of air and water, so nearly the same the world over. These are still far most numerous in individuals, and have, so to say, the surest hold on life. When we think of the vast void below which the “improvement” out of their sphere of these would leave, and the increased risks which complicated structure (in machines or beings) has to run, we shall not wonder that the simple still numerically predominate. TO CHARLES DARWIN. CAMBRIDGE, June 10, 1877. MY DEAR MR. DARWIN,--Except when you are to be aided in your work I decline to give letters of introduction to you, knowing how you are occupied and how infirm your health at any time may be. So please take this note to mean just this. The happy couple who bear it would be delighted to call some day, if you say so, and pay their respects to you, and I will tell you why I am disposed to promote their wishes.