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Despite all these successes, Darwin knew that his work was missing a very important link. His most famous theory, which (to be succinct) stated that species evolve by passing on advantageous traits, could not explain how the traits were actually passed on (Kean). In his book The Variation of Animals and Plants under Domestication, Darwin was blunt in stating that he "ha[d] been led, or rather forced, to adopt a view which to a certain extent connects these facts by a tangible method" (Winther).
Since Gregor Mendel, the patriarch of genetic inheritance, was still known mostly as a monk¬, in other words, not known very much at all, Darwin was all alone in his search for an inheritance mechanism.
He started out on the right track; he realized that, when a trait reappeared suddenly after being absent for generations, it had likely been stored in a dormant state through those generations instead of being spontaneously recreated (Winther). His problem was in his proposed method of storing the traits.
This theory was very intricately crafted over at least 24 years to correspond with Darwin's widely accepted theory of evolution; however, it was contradicted by, among other things, the cell theory and direct experimentation by Darwin's cousin Francis Galton. In spite of these difficulties, Darwin did not want to let go of it, in one correspondence referring to it as "The great god Pan" (Schwartz). This is the real problem; science is supposed to give weight to ideas or not depending solely on their objective validity, not the amount of effort that was put into their creation or their necessity in holding up another idea. Darwin fell into the trap of what Michael Shermer has identified as a primary problem in holding beliefs both in and out of science: he formulated the belief first, and followed with the justification. The scientific method is grounded in the opposite process, formulating unbiased beliefs out of objective facts. The fact that one of the most revered scientists of the human race made such an egregious error shows that the scientific community is often prone to losing sight of its values.
1The fact Darwin proposes two methods here may indicate that this part of the gemmule theory was a bit forced. In fact, it got him into trouble when the cell theory was developed, which states that all cells come from other cells, implying that they do not come from gemmules combining with either other gemmules or other cells.
References:
Bartley, Mary M. "Darwin and Domestication: Studies on Inheritance." Journal of the History of Biology (1992).
Kean, Sam. "The Violinist's Thumb." Brown (2012).
Montgomery, Stephen. "Darwin & Botany." Christ's College, U of Cambridge (2009).
Schwartz, James. "In Pursuit of the Gene: From Darwin to DNA." Harvard U (2008).
Shermer, Michael. "The Believing Brain." St. Martin's Griffin (2011).
Winther, Rasmus G. "Darwin on Variation and Heredity." Journal of the History of Biology (2000).
Image Credit: The Hornet (Wikipedia)