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In Darwin's time, biologists held to the theory of blending inheritance—an offspring was an average of its parent. If an individual had one short parent and one tall parent, it would always be of some interim height between that of its parents. In turn, this interim height would become a theoretical extreme (either tall or short) that, in turn, bounded the potential height of its own offspring. If this were the case, all hereditary variations would be diluted out of a population in a few generations, rather than the beneficial variations accumulating, as the theory of evolution, and simple observations, required.

Example of blending inheritance using the color of flowers to show how a species color variation would converge upon one color in relatively few generations if its offspring's color variations were truly bounded by the parent's colors.

[edit] Origins

Historically, this idea was little more than one of several short-lived 19th century "arm-chair" biological hypotheses that attempted to explain inheritance in an era before genetics, and evidence was limited to simple observation of variations between generations. It was widely circulated but never officially authored. This idea was abandoned after the experiments of Gregor Mendel became public. It is similar to the modern idea of incomplete dominance, in that the offspring will express a blending of the different traits of the parents. However, incomplete dominance results in blending only of the phenotype, keeping the alleles within the heterozygote distinct (and, thus still inheritable in successive generations), whereas the theory of blending inheritance referred to an actual blending of the genetic material (i.e. in modern terms, alleles would blend together to form a completely new allele).

In this particular hypothesis, the offspring's trait, from parents with differing traits, could express itself exactly like either trait from one of the two parents or could express itself as some intermediate mixing of the parental traits. This blending theoretically was a smooth spectrum of infinite possible outcomes. This theory asserts inheritance worked similar to the way paint mixes. And, like with paint, where two colors, once mixed, become unrecoverable via successive mixing with still other colors, the historical theory of blending inheritance required that at least half of all genetic variances to be completely unrecoverable in successive generations, soon resulting in a complete loss of all variation.

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