What term did Darwin use to refer to the units of inheritance that could be passed on to offspring?

Darwin introduced the term "gemmules" to explain his theory of inheritance. He proposed that gemmules were tiny particles produced by all parts of the body that carried information about traits and were thought to collect in the reproductive organs to be passed on to offspring. This concept was part of his attempt to understand how characteristics were transmitted from parents to offspring before the modern understanding of genetics and DNA.

In contrast, "alleles" are variations of a gene that arise due to mutations and contribute to the genetic diversity in a population. "Genotypes" refer to the genetic constitution of an individual, while "phenotypes" describe the observable characteristics of an organism. While these terms are important in genetics, they do not correspond to the specific term used by Darwin to describe units of inheritance. The term "gemmules," while later found to be inaccurate in the context of modern genetics, is thus correctly identified as the concept he proposed for this purpose.