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That his principles of acceleration and retardation, his old age theory, and other views of the stages of life are not yet fully grasped, is true. Many have not understood them. Though they are all evolutionary, yet Darwin himself was perplexed. I may be permitted to give an extract from a letter which Darwin wrote to me concerning this very matter. In this letter he asks, "What is the meaning of Professor Cope's and Hyatt's views on acceleration and retardation? I have endeavored and given up in despair an attempt to grasp their meaning"

[The original letter, as published in Correspondence vol. 25, p. 171 reads "There is one point which I regret that you did not make clear in your Address,—namely what is the meaning & importance of Prof. Cope & Hyatts views on Acceleration & Retardation: I have endeavoured & given up in despair the attempt to grasp their meaning.—" The editors note: "Edward Drinker Cope and Alpheus Hyatt promoted a theory of evolution based on acceleration and retardation of development. CD mentioned their work in Origin 6th ed., p. 149. For CD's views and uncertainty on the topic, see Correspondence vol. 20, letter to Alpheus Hyatt, 10 October [1872]. For more on the background and development of Cope's and Hyatt's ideas, see S. J. Gould 1977, pp. 85–96."]