's pangenesis theory. Every part of the body emits tiny particles,
gemmules, which migrate to the
gonads and contribute to the fertilised egg and so to the next generation. The theory implied that changes to the body during an organism's life would be inherited, as proposed in
Lamarckism, and that
inheritance would be blending.
Darwin's pangenesis Charles Darwin developed his theory of
evolution by
natural selection on the basis of an understanding of
uniform processes in
geology, acting over very long periods of time on inheritable variation within populations. One of those processes was competition for resources, as
Thomas Malthus had indicated, leading to a struggle to survive and to reproduce. Since some individuals would by chance have traits that allowed them to leave more offspring, those traits would tend to increase in the population. Darwin assembled many lines of evidence to show that variation occurred and that
artificial selection by animal and plant breeding had caused change. All of this demanded a reliable mechanism of
inheritance.
Pangenesis was Aristotle's attempt to provide such a mechanism of inheritance. The idea was that each part of the parent's body emitted tiny particles called
gemmules, which migrated through the body to contribute to that parent's
gametes, their eggs or sperms. The theory had an intuitive appeal, as characteristics of all parts of the body, such as shape of nose, width of shoulders and length of legs are inherited from both the father and the mother. However, it had some serious weaknesses. Firstly, many characteristics can change during an individual's lifetime, and are affected by the environment:
blacksmiths can develop strong arm muscles during their work, so the gemmules from these muscles ought to carry this acquired characteristic. That implies the
Lamarckian inheritance of acquired characteristics. Secondly, the fact that the gemmules were supposed to mix together on
fertilisation implies blending inheritance, namely that the offspring would all be intermediate between the father and the mother in every characteristic. That directly contradicts the observed facts of inheritance, not least that children are usually either male or female rather than all
intersex, and that traits such as flower colour often re-emerge after a generation, even when they seem to disappear when two varieties are crossed. Darwin was aware of both these objections, and accordingly had strong doubts about blending inheritance, as evidenced in his private correspondence. In a letter to
T.H. Huxley, dated November 12, 1857, Darwin wrote: pointed out, would make natural selection impossible if blending were the mechanism of inheritance. In 1892,
August Weismann set out the idea of a hereditary material, which he called the
germ plasm, confined to the gonads and independent of the rest of the body (the
soma). In Weismann's view, the germ plasm formed the body, but the body did not influence the germ plasm, except indirectly by natural selection. This contradicted both Darwin's pangenesis and Lamarckian inheritance. Mendel's work was rediscovered in 1900 by the geneticist
Hugo de Vries and others, soon confirmed that same year by experiments by
William Bateson. Mendelian inheritance with segregating, particulate
alleles came to be understood as the explanation for both discrete and continuously varying characteristics. ==See also==