An argument used against
Darwin's theory of
evolution in the 18th century.
The gist of this argument is that paternal and maternal contributions for an offspring would be eventually diluted by breeding with statistically normal examples of the species.
We now know, of course, that genetic contributions by ancestors leads to the degree of variation possible, and that in this way, a variation on a species norm will (if procreatively successful) be perpetuated throughout the species.