Andrew Knight saw the truth much more clearly, for he remarks, “Nature intended that a sexual intercourse should take place between neighbouring plants of the same species.” (1/5.  ’Philosophical Transactions’ 1799 page 202.) After alluding to the various means by which pollen is transported from flower to flower, as far as was then imperfectly known, he adds, “Nature has something more in view than that its own proper males would fecundate each blossom.”  In 1811 Kolreuter plainly hinted at the same law, as did afterwards another famous hybridiser of plants, Herbert. (1/6.  Kolreuter ’Mem. de l’Acad. de St. Petersbourg’ tome 3 1809 published 1811 page 197.  After showing how well the Malvaceae are adapted for cross-fertilisation, he asks, “An id aliquid in recessu habeat, quod hujuscemodi flores nunquam proprio suo pulvere, sed semper eo aliarum suae speciei impregnentur, merito quaeritur?  Certe natura nil facit frustra.”  Herbert ’Amaryllidaceae, with a Treatise on Cross-bred Vegetables’ 1837.) But none of these distinguished observers appear to have been sufficiently impressed with the truth and generality of the law, so as to insist on it and impress their beliefs on others.

In 1862 I summed up my observations on Orchids by saying that nature “abhors perpetual self-fertilisation.”  If the word perpetual had been omitted, the aphorism would have been false.  As it stands, I believe that it is true, though perhaps rather too strongly expressed; and I should have added the self-evident proposition that the propagation of the species, whether by self-fertilisation or by cross-fertilisation, or asexually by buds, stolons, etc. is of paramount importance.  Hermann Muller has done excellent service by insisting repeatedly on this latter point.

It often occurred to me that it would be advisable to try whether seedlings from cross-fertilised flowers were in any way superior to those from self-fertilised flowers.  But as no instance was known with animals of any evil appearing in a single generation from the closest possible interbreeding, that is between brothers and sisters, I thought that the same rule would hold good with plants; and that it would be necessary at the sacrifice of too much time to self-fertilise and intercross plants during several successive generations, in order to arrive at any result.  I ought to have reflected that such elaborate provisions favouring cross-fertilisation, as we see in innumerable plants, would not have been acquired for the sake of gaining a distant and slight advantage, or of avoiding a distant and slight evil.  Moreover, the fertilisation of a flower by its own pollen corresponds to a closer form of interbreeding than is possible with ordinary bi-sexual animals; so that an earlier result might have been expected.