Certain naturalists have recently laid too great stress, as it appears to me, on the difference in fertility between varieties and species when crossed. Some allied species of trees cannot be grafted on one another, whilst all varieties can be so grafted. Some allied animals are affected in a very different manner by the same poison, but with varieties no such case until recently was known; whilst now it has been proved that immunity from certain poisons sometimes stands in correlation with the colour of the individuals of the same species. The period of gestation generally differs much in distinct species, but with varieties until lately no such difference had been observed. Here we have various physiological differences, and no doubt others could be added, between one species and another of the same genus, which do not occur, or occur with extreme rarity, in the case of varieties; and these differences are apparently wholly or in chief part incidental on other constitutional differences, just in the same manner as the sterility of crossed species is incidental on differences confined to the sexual system. Why, then, should these latter differences, however serviceable they may indirectly be in keeping the inhabitants of the same country distinct, be thought of such paramount importance, in comparison with other incidental and functional differences? No sufficient answer to this question can be given. Hence the fact that widely distinct domestic varieties are, with rare exceptions, perfectly fertile when crossed, and produce fertile offspring, whilst closely allied species are, with rare exceptions, more or less sterile, is not nearly so formidable an objection as it appears at first to the theory of the common descent of allied species. CHAPTER 2.XX. SELECTION BY MAN. SELECTION A DIFFICULT ART. METHODICAL, UNCONSCIOUS, AND NATURAL SELECTION. RESULTS OF METHODICAL SELECTION. CARE TAKEN IN SELECTION. SELECTION WITH PLANTS. SELECTION CARRIED ON BY THE ANCIENTS AND BY SEMI-CIVILISED PEOPLE. UNIMPORTANT CHARACTERS OFTEN ATTENDED TO. UNCONSCIOUS SELECTION. AS CIRCUMSTANCES SLOWLY CHANGE, SO HAVE OUR DOMESTICATED ANIMALS CHANGED THROUGH THE ACTION OF UNCONSCIOUS SELECTION. INFLUENCE OF DIFFERENT BREEDERS ON THE SAME SUB-VARIETY. PLANTS AS AFFECTED BY UNCONSCIOUS SELECTION. EFFECTS OF SELECTION AS SHOWN BY THE GREAT AMOUNT OF DIFFERENCE IN THE PARTS MOST VALUED BY MAN.