The History of Science Society Charles Darwin and Blending Inheritance Author(s): Peter Vorzimmer Source: Isis, Vol. 54, No. 3 (Sep., 1963), pp. 371-390 Published by: The University of Chicago Press on behalf of The History of Science Society Stable URL: http://www.jstor.org/stable/228805 Accessed: 11/10/2009 01:15 Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/action/showPublisher?publisherCode=ucpress. Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. The University of Chicago Press and The History of Science Society are collaborating with JSTOR to digitize, preserve and extend access to Isis. http://www.jstor.org Charles Darwin and Blending Inheritance By Peter Vorzimmer * I. INTRODUCTION THE evolutionary writings of Charles Darwin (d. 1882), in the latter part of his life, when contrasted with the Origin of Species of 1859, indicate a considerable change in his evolutionary thought over the inter- vening years. Unfortunately, despite a large amount of Darwiniana pub- lished during the centennial years, there has been a noticeable lack of information on Darwin's post-Origin development. Since the over-all change effected appears great and the resultant view nearly antithetical to that of the first edition of the Origin, it has been felt that such a radical change could not be looked upon merely as a modification of an earlier view, but as an adoption of a distinctly new one. It is on this assumption that recent writers, in glancing over the post-Origin period, have attempted to ascertain when and why Darwin made such a significant change. A number of these authors 1 in searching for a turning point in Darwin's evolutionary thought - a point at which he was forced to revert to once- rejected Lamarckian mechanisms - have fastened upon the attack made by Fleeming Jenkin in 1867. In fact, it may be said that this represents a con- sensus of opinion among those writers to date who have discussed this ques- tion. One reason for this appears to lie in the fact that Jenkin's name alone stands out in the available published material as a critic employing against natural selection the swamping effect of blending inheritance. Furthermore, the appearance of Jenkin's review 2 immediately before Darwin undertook the great revisions appearing in the fifth Origin 3 also suggested some causal connection. These facts seemed to be corroborated by evidence from Darwin himself that " Fleeming Jenkins [sic] has given me much trouble . Fleem- ing Jenkin's arguments have convinced me " 4 and together they presented a * Cambridge University. 3 60% of all of Darwin's revisions on the Note: In the citations coming from the Dar- Origin occurred in the last two editions, 21% win Reprint Collection at the Botany School in the fifth alone. From Morse Peckham, The Library, Cambridge, all quotations are passages Origin of Species: A Variorum Text, 1959, marked by Darwin unless otherwise noted. (Philadelphia: University of Pennsylvania 1 G. Hardin, Nature and Man's Fate, 1959, Press). (New York: Reinhart); L. Eiseley, Darwin's 4 (Ed.) Francis Darwin, More Letters of Century, 1959, (London: Gollancz); P. Fother- Charles Darwin, 1903, (2 vols., New York: gill, Historical Aspects of Organic Evolution, Appleton) Vol. II, p. 379; (Ed.) Francis Dar- 1952, (London: Hollis and Carter). win, Life and Letters of Charles Darwin, 1898, 2 Fleeming Jenkin, " The Origin of Species " (2 vols., New York: Appleton) Vol. II, p. 288 in The North British Review, Vol. 46, June, (Hereafter More Letters and Life and Letters). 1867. 371 ISIS, 1963, VOL. 54, PART3, No. 177. 372 PETER VORZIMMER convincing case for establishing Jenkin as the major cause for the change that followed. In arriving at this conclusion in the absence of sufficient research, these writers have by implication asserted a great deal of misin- formation regarding both genetic and evolutionary thought at this time. Contained in their case are the following assumptions: 1. That, while not the first writer to note the swamping implications of blending inheritance, Fleeming Jenkin was the first to bring this to bear against natural selection. 2. That Jenkin's essay was the first employing this criticism to be acknowledged by Darwin and which he (Darwin) felt posed some serious problems. 3. That the effect on Darwin of Jenkin's review was to bring about a great reassessment in his thought, the result of which was a staggering reduction of the power of natural selection. 4. That the impact of Jenkin's criticism was due to the fact that Darwin had previously overlooked the problem of blending. 5. That the criticism forced Darwin to create a Lamarckian theory of inheritance - his " Pangenesis " of 1868. It is on the basis of these assumptions that, for these writers, the story of Charles Darwin and his struggle with the implications of blending in- heritance begins and ends with Fleeming Jenkin. They have been led to look upon Jenkin's criticism as being of such originality and validity (ex- aggerated by Darwin's supposed oversights), that they have felt in it an exigency which Darwin never did nor could have. In the light of recent research into the whole of the post-Origin period (1859-1882),5 these assumptions can be seen to be incorrect. All these historical deductions hinge upon a mistaken impression as to the nature of Darwin's post-Origin change. There is no turning point in Darwin's evolu- tionary thought. What unrolls before us in the twenty-three years after the Origin is a gradual but progressive modification.6 As for Jenkin and the place of his swamping criticism, it is hoped that this article will indicate their true position while describing ab initio the place of the concept of blending inheritance in the development of Darwin's thought. II. BLENDING INHERITANCE The term blending inheritance refers to the hereditary mixing of both paternal and maternal elements in the offspring in such a way as to give the outward appearance that both have blended into an inseparable mixture and present, in the offspring, a feature which appears to be mid-way between the two. Such a view is merely a description of what can be observed in most crosses on a purely superficial level. It is so common as to be considered 5 See P. J. Vorzimmer, The Development of 6 Darwin started to modify the Origin be- Darwin's Evolutionary Thought after 1859, un- fore the first edition had gone on sale. The published doctoral dissertation, Cambridge roots of nearly all the significant changes can University, 1963. be seen in the first edition. CHARLES DARWIN AND BLENDING INHERITANCE 373 universal. In fact it was, to many nineteenth-century naturalists, the rule - with a few less noticeable exceptions. The " swamping effect " occurs under blending inheritance if one or a few aberrant organisms arise within the normal population. Since these rare variant forms must breed back into the general population, the new and " unusual characters will be " swamped by being absorbed into and blended with a vast pool of normal characters. The concept of blending inheritance as a natural process, together with all its implications, has also been called "The Paint-Pot Theory of He- 7 redity." The analogy is that the normal population is likened to a bucket of white paint and the variant forms to a few drops of black. The effect of mixing the two paints is analogous to the effect of free intercrossing in Nature. Additionally, the impossibility of separating two once-distinct fluids after mixing is carried over as the impossibility of natural selection accumu- lating small changes under blending inheritance. III. BLENDING INHERITANCE BEFORE JENKIN (1867) A. General Remarks As has already been pointed out, blending inheritance is not so much an hypothesis as an empirical generalization. A blending of two distinct kinds is readily observed throughout organic nature (in the process of sexual reproduction). The first is that the offspring appears to consist of a con- " glomerate mixture of features from both parents.8 The second or true blending" occurs when the offspring can be seen to represent, in terms of a single feature, an equal blend of the two distinct parental characteristics for that feature. The latter of these two kinds of mixing forms the basis for the " swamping effect." Both of these types of hereditary mixing had been observed for centuries before the time of Darwin and Jenkin. By the time the study of hereditary 9 phenomena became a scientific discipline in the last quarter of the eight- eenth century, blending in inheritance had become accepted as axiomatic. Yet at about this time, two related phenomena were also recognized, and both were obvious exceptions to blending as an all-inclusive or even pre- dominant process.