Sent C.B. Davenport to K. Pearson, 5th June 1903.
5 Jun 1903
Description:

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Now I will take up some of the matters to which you specifically allude. “Have you any clear conception of difference between an extreme normal variation and a mutation?” I have formulated the conception of a mutation qualitative and a mutation quantitative.

A qualitative mutation would be one which a new quality, not in the normal type, appears. The new arrangement of branches or leaves on the stem of fasciated plants is an example. Such fasciation crops out in a great variety of species of plants, and even in nature a colony of fasciated plants may occur, as in the case of a colony of fasciated herbs in the Mission River. Syndactylism and polydactylism are other examples. There is more than one syndactylic “species” (e.g. Hylobates syndactylic. See Lewis Synopsis I. p. 157.[)]

Quantitative mutations are those in which the mutating individuals constitute, with reference to the mutating of character, a new frequency polygon outside the ordinary range of the main frequency polygon. Say for the normal frequency polygon beyond 60. Of course, the fact that you get one isolated individual at 60 may have not importance. Such scattered individuals one always finds at the extremes and they are to be expected by theory as you have shown. With sufficiently rigid selection such an extreme case might become a modal individual.

As to Darwin’s position in respect to mutation, I think a single quotation from a letter may do injustice to the opinion of so many sided a man and one who knew too much to think that there was only one method of evolution or to think so all the time. De Vreis (Mutant, Theorie[?] pp. 20-28) shows how Darwin considered mutations to play an important point in the Origin of Species. Wholly apart from what Darwin or others think on the subject the data that I have been collecting for 12 years on this subject has convinced me of the importance of mutations in the production of specific differences.

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It seems to me that the experimental evidence that is lacking is not for mutation or for direct action of environment as factors of a specific change, but for selection of the slight individual variations as such a factor. Weldon’s work on crabs seems to me the best but even there the effect wrought, was not great (although the time was certainly short), and we don’t yet know how great a modification will be eventually produced. De Vries’ experiments are certainly instructive as to the increasing difficulty of producing modifications as we depart farther & farther from the modal condition, and my agricultural friends state that they have the same experience.

You say that I practically throw Darwin over & adopt a mutation theory. I do not lay great stress on Natural Selection because this has, evidently, as originally considered by Darwin, little or nothing to do with variation. In the summary, of Chapter IV, Orig. of Species, 6^th ed. he says: Considering the infinite complexity of the relations of all organic beings to each other & to their conditions of life, causing [NB. conditions cause; not nat. select.] an infinite diversity in structure, constitution, & habits, to be advantageous to them, it would be a most extraordinary fact, if no variation has ever occurred useful to each being’s own welfare... But if variation useful to any organic beings do occur, assuredly individuals thus characterized will have the best chance of being preserved in the struggle for life.” &c. True Darwinism is a theory of adaptation. Darwin taught that variations (including, I think, also varieties) were due to direct influence of environment; is it not you rather than I who would throw Darwin over?

Finally, you think to accept the possibility of mutation (as Darwin did) and to insist on the great importance of the direct modifying effects of environment (as Darwin most emphatically did) stultifies my paper & biometric work in general. This astounds me greatly. In your Grammar of Science p. 379 after an analysis of the sources of change you conclude, very judiciously, that satisfactory numerical demonstration of its existence (inheritance of the [illeg.]) is wanting and then go on to show how evolution can take place by selection alone. Did you pin your faith in the importance of biometry in the absence of that demonstration? And will you abandon it if the demonstration shall be satisfactorily made tomorrow? I would not. Biometry is numerical precision and that is the soul of science, according to Herschel.

Well, I have written a long letter in my plea that Biometrika be not run in the interest of one idea but that everyone who has used the methods of numerical precision properly and will adopt the matter[?]; “Ignoramus, in hoc sigus[?] laboremus[?]” may have full entry to it to express the truth as he sees it.’