...a cross between different varieties, or between individuals of the same variety but of another strain, gives vigour and fertility to the offspring; and on the other hand, that close interbreeding diminishes vigour and fertility...no organic being self-fertilises itself for an eternity of generations; but that a cross with another individual is occasionally - perhaps at very long intervals - indispensable.So...sex is necessary for improved vigour in a species...and what happened before species that had sexual reproduction? Or indeed with species that do not reproduce sexually now? Do amoeba not just split? How is it that they continue to have "vigour" and "fertility"?
Further on Darwin mentions that he has not found one land animal that is perfectly hermaphroditic, and that although some marine animals are, that water currents can provide the means by which the occasional cross-fertilisation can take place. He mentions one species that he had initially had problems with - Cirripedes - but he found evidence that it, too, sometimes crosses.
I am still not sure about amoeba and the like - is there any chance by which they can cross-fertilise?
A final quote from Darwin on this:
Finally, then, we may conclude that in many organic beings, a cross between two individuals is an obvious necessity for each birth; in many others it occurs perhaps only at long intervals; but in none, as I suspect, can self-fertilisation go on for perpetuity.