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It is, indeed obvious that the presence of gemmules in the blood can form no necessary part of my hypothesis; for I refer in illustration of it to the lowest animals, such as the Protozoa, which do not possess blood or any vessels; and I refer to plants in which the fluid, when present in the vessels, cannot be considered as true blood.
There is no evidence that anyone again attempted an experiment to determine the veracity of pangenesis and its messenger gemmules.
If pangenesis operated properly and the gemmules were transferred, he predicted that the offspring from those rabbits would not breed true.
This is also a wonderful example of what Karl Popper (1963) called "conjecture and refutation," an intellectual process whereby ideas related to the underlying principle (in this case the gemmules of pangenesis) are proposed and then subjected to experiment in an attempt to reject them.
Darwin suggested that the gemmules were physical entities and that, as such, other scientists could look for them or for their effects.
Gemmules were lightly visible throughout the sample and were brown to black in color (Figure 1, left).
Hatching of freshwater sponge gemmules after low temperature exposure: Ephydatia mulleri (Porifera: Spongillidae).