Figure 6

This notion of some latent “tendency” departed manifestly from normal blending heredity, and in many ways it was close in spirit to Mendelian heredity. Yet it apparently did not occur to Darwin, at least initially, to invoke this idea of latency in his struggle to respond to Jenkin. Instead, Darwin decided to change the emphasis from the role he had previously assigned to single variations to that of individual differences (the wide spectrum of tiny differences occurring frequently, which was supposed to be distributed continuously throughout the population), in supplying the “raw materials” for natural selection to effect. In other words, Darwin now relied on an entire continuum of variations for the production of evolution by natural selection over many generations.

In a letter to Wallace on January 22, 1869, the distressed Darwin wrote, “I have been interrupted in my regular work in preparing a new edition of the ‘Origin,’ which has cost me much labour, and which I hope I have considerably improved in two or three important points. I always thought individual differences more important than single variations, but now I have come to the conclusion that they [individual differences] are of paramount importance, and in this I believe I agree with you. Fleeming Jenkin’s arguments have convinced me.” To reflect his new emphasis, Darwin amended the fifth edition and subsequent editions of The Origin by changing singulars referring to individuals into plurals, as in “any variation” turning into “variations,” and “an individual” into “individual differences.” He also added a few new paragraphs in the fifth edition, two of which, in particular, are of great interest. In one, he admitted openly:

I saw, also, that the preservation in a state of nature of any occasional deviation of structure, such as a monstrosity, would be a rare event; and that, if preserved, it would generally be lost by subsequent intercrossing with ordinary individuals. Nevertheless, until reading an able and valuable article in the “North British Review” (1867), I did not appreciate how rarely single variations, whether slight or strongly marked, could be perpetrated.