The offspring of parents of two different races is called a mongrel, whilst the term "hybrid" has been of late limited, for the sake of convenience, to the offspring of parents of two different species. Mongrels, it has been generally held, are fertile–often more fertile than pure-bred individuals whose parents are both of the same race, whilst "hybrids" are contrasted with them, in being infertile. We have seen that infertility is not an absolute rule in the case of hybrids, and it appears that there is also a source of error in the observations which lead to the notion that "mongrels" are always fertile. The fact is that observations on this matter have nearly always been made with domesticated animals and plants which are, of course, selected and bred by man on account of their fertility, and thus are exceptionally characterized by fertility, which is transmitted in an exceptional degree to the races or varieties which are experimentally inter-bred, and, consequently, may be expected to produce fertile mongrels. Alfred Russel Wallace insisted upon this fact, and pointed out that in a few cases colour varieties of a given species of plant have been found to be incapable of inter-breeding, or only produce very few "mongrels." This has been established in the case of two dissimilarly-coloured varieties of mullein. Also the red and the blue pimpernel (the poor man's weather-glass, Anagallis), which are classed by botanists as two varieties of one species, have been found after repeated trials to be definitely incapable of inter-breeding. Wallace insists in regard to crossing, that some degree of difference favours fertility, but a little more tends to infertility. We must remember that the fertility of both plants and animals is very easily upset. Changed conditions of life–such as domestication–may lead (we do not know why) to complete or nearly complete infertility; and, again, "change of air," or of locality, has an extraordinary and not-as-yet-explained effect on fertility. "Oh, the little more and how much it is! And the little less, and what worlds away!"