The examples given may suffice to convey a general idea of the phenomenon, ordinarily called atavism by gardeners, and considered mostly to be the effect of some innate tendency to revert to the ancestral form. It is on this conception that the almost universal belief rests, that varieties are distinguished, as such, from species by their inconstancy. Now I do not deny the phenomenon itself. The impurity of seeds and cultures is so general and so manifest, and may so easily be tested by every one [199] that it cannot reasonably be subjected to any doubt. It must be conceded to be a fact, that varieties as a rule revert to their species under the ordinary circumstances of commercial culture. And I cannot see any reason why this fact should not be considered as stating a principal difference between varieties and species, since true species never sport into one another. My objection only refers to the explanation of the observed facts. According to my view nearly all these ordinary reversions are due to crosses, and it is for this reason that I proposed to call them by a separate name, that of "vicinists." Varieties then, by means of such spontaneous intercrossing sport into one another, while species either do not cross, or when crossing produce hybrids that are otherwise constituted and do not give the impression of atavistic reversion. I must not be content with proposing this new conception, but must give the facts on which this assumption rests. These facts are the results of simple experiments, which nevertheless are by no means easy to carry out, as they require the utmost care to secure the absolute purity of the seeds that are employed. This can only be guaranteed by previous cultures of isolated plants or groups of plants, or by artificial pollination. [200] Once sure of this preliminary condition, the experiment simply consists in growing a variety at a given distance from its species and allowing the insects to transfer the pollen. After harvesting the seed thus subjected to the presumed cause of the impurities, it must be sown in quantities, large enough to bring to light any slight anomaly, and to be examined during the period of blooming.