Blending Inheritance - History

History

The obvious shortcomings, such as those mentioned above, with the blending inheritance model were not completely lost to every 19th century thinker. In fact, these inadequacies made for an atmosphere in which many lesser-known, and equally unconvincing, 19th century "arm-chair" hypotheses to be formulated and circulated in attempts to explain inheritance more adequately (see inheritance of acquired characters, maternal impression, telegony, preformationism, Geoffroyism, Pangenesis). It took the experiments of Gregor Mendel, presented in Experiments on Plant Hybridization, to finally provide a better model than the one proposed by blending inheritance, and to dismiss the myriad of other speculative ideas erupting at this time. Mendel discredited blending inheritance theory by proposing the theory of particulate inheritance.

Darwin himself also had strong doubts of the blending inheritance hypothesis, despite incorporating a limited form of it into his own explanation of inheritance published in 1868, called pangenesis. Not least of all, his objections likely arose because it conflicted with his own theory of natural selection. This incompatibility was also noted by a contemporary critic of Darwin's, Fleeming Jenkin, in a now infamous excoriation of Darwin's Origin of Species. Jenkin, a strong proponent of the blending inheritance idea, used blending inheritance to argue against the plausibility of natural selection itself. If one was to assume that blending inheritance was at work, Jenkins argued that any favorable trait that might arise in a lineage for which natural selection could possibly work upon, would naturally be blended away from that lineage long before the much slower processes of natural selection could act upon it and improve it.

Moreover, prior to Jenkin, Darwin expressed his own distrust of blending inheritance to both T.H. Huxley and Alfred Wallace. In a letter to Wallace, dated February 6, 1866 (coincidentally, this was the same year Mendel formally published the aforementioned article), Darwin mentioned conducting hybridization experiments very similar to Mendel's, with pea plants no less, to prove to himself that blending inheritance did not work as a model for inheritance in certain varieties of species:

"... I do not think you understand what I mean by the non-blending of certain varieties. It does not refer to fertility; an instance I will explain. I crossed the Painted Lady and Purple sweetpeas, which are very differently coloured varieties, and got, even out of the same pod, both varieties perfect but not intermediate. Something of this kind I should think must occur at least with your butterflies & the three forms of Lythrum; tho’ those cases are in appearance so wonderful. I do not know that they are really more so than every female in the world producing distinct male and female offspring..."

Also, in an earlier letter to Huxley, Dated November 12, 1857, nearly nine years before the letter to Wallace, Darwin expressed his beginnings of doubt regarding the idea of blending inheritance:

"I have lately been inclined to speculate very crudely & indistinctly, that propagation by true fertilisation, will turn out to be a sort of mixture & not true fusion, of two distinct individuals, or rather of innumerable individuals, as each parent has its parents & ancestors:— I can understand on no other view the way in which crossed forms go back to so large an extent to ancestral forms."