Darwin's theory of pangenesis

For reference, below is a quote from the 2nd ed. of Darwin's Variation (v. 2, 457), summarizing his theory of pangenesis:

"It is universally admitted that the cells or units of the body increase by self-division or proliferation, retaining the same nature, and that they ultimately become converted into the various tissues and substances of the body. But besides this means of increase I assume that the unites throw off minute granules which are dispersed throughout the whole system; that these, when supplied with proper nutriment, multiply by self-division, and are ultimately developed into units like those from which they were originally derived. These granules may be called gemmules. They are collected from all parts of the system to constitute the sexual elements, and their development in the next generation forms a new being; but they are likewise capable of transmission in a dormant state to future generations and may then be developed. Their development depends on their union with other partially developed or nascent cells which precede them in the regular course of growth...Gemmules are supposed to be thrown off by every unit, not only during the adult state, but during each stage of development of every organism; but not necessarily during the continued existence of the same unit. Lastly, I assume that the gemmules in their dormant state have a mutual affinity for each other, leading to their aggregation into buds or into the sexual elements. Hence, it is not the reproductive organs or buds which generate new organisms, but the units of which each individual is composed. These assumptions constitute the provisional hypothesis, which I have called Pangenesis."

When reading the Origin, keep in mind that Darwin developed his theory of pangenesis many years after he first developed and published the idea of evolution by natural selection.

Galton conducted blood transfusion experiments to test whether gemmules were carried in the blood. Galton and Darwin corresponded extensively about the design of these experiments and their results. Darwin was clearly reluctant to believe the negative results, and ended up criticizing the experiments in a letter to Nature after the results were published. That Darwin would criticize after the fact experiments that he heartily approved of before they were conducted is widely viewed as a rare instance of Darwin closing his mind to relevant data (and acting like a bit of a jerk).

Also, keep in mind that Mendel himself didn't actually propose an underlying mechanism to explain the statistical patterns of inheritance he observed. The aim of Darwin's theory of pangenesis was to serve as a concrete mechanism that would explain both transmission (inheritance) and development. To our modern eyes, Mendel's data cry out for an explanation in terms of genes on chromosomes--the mechanistic explanation almost seems to jump out of the data themselves. But that's not how things would've appeared to Darwin, had he been aware of Mendel's data. It's hard to say exactly how knowledge of Mendel's work might have affected Darwin's thinking.