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Show 190 HYBRIDISM. CHAP. XIX. duced. Thus, as I believe, we can understand why with domesticated animals varieties have not been produced which arc mutually sterile; and why with plants only a few such cases have been observed, namely, by Gartner, with certain varieties of maize and verbascum, by other experimentalists with varieties of the gourd and melon, and by Kolreutcr with one kind of tobacco. With respect to varieties which have originated in a state of nature, it is almost hopeless to expect to prove by direct evidence that they have been rendered mutually sterile; for if even a trace of sterility could be detected, such varieties would at once be raised by almost every naturalist to the rank of distinct species. If, for instance, Gartner's statement were fully confirmed, that the blue and red-flowered forms of the pimpernel (Anagallis arvensis) are sterile when crossed, I presume that al] the botanists who now maintain on various grounds that these two forms are merely fleeting varieties, would at once admit that they were specifically distinct. The real difficulty in our present subject is not, as it appears to me, why domestic varieties have not become mutually infertile when crossed, but why this has so generally occurred with natural varieties as soon as they have been modified in a sufficient and permanent degree to take rank as species. We are. far from precisely ~nowing the cause; nor is this surprising, seemg how profoundly 1gnorant we are in regard to the normal and abnormal action of the reproductive system. But we can see that species, owing to their struggle for life with numerous competitors, must have been exposed to more uniform conditions during long periods of time, than have been domestic varieties· and this may well make a wide difference in the result. For w~ know how commonly wild animals and plants, when taken from thei:· natural conditions and subjected to captivity, are rendered stenle; and t_he reproductive functions of organic beings which have always hved and been slowly modified under natural c:onditio. ns would probably in like manner be eminently sensitive to the mfluence of an unnatural cross. Domesticated productions on the. oth.er hand, which, as shown by the mere fact of theil: ~omes:10at10n~ ~vere not originally highly sensitive to changes m theu cond1twns of life, and w.hich can now generally resist CHAP. XIX. HYBRIDISM. 191 w~th undiminished fertility repeated changes of conditionA, m1ght be expected to produce varieties, which would be little liable to have their reproductive powers injuriously affected by the act of crossing with other varieties which had originated in a like manner. Certain naturalists have recently laid too great stress1 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 each other,-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, but now 1t has been prov~d that immunity from certain poisons stands in some cases in correlation with the colour of the hair. The period of gestation generally differs much with distinct species, but with varieties until lately no such difference had been observed. The time required for the germination of seeds differs in an analogous manner, and I am not aware that any difference in this respect has as yet been det ected with varieties. Here we have various physiological differences, and no doubt others could be added, between one species and another o£ the same genus, which do not occur, or occur with extreme rarity, in the case of varieties; and these differences are apparent]y 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, howe: er serviceab]e they may indirectly be in keeping the inhabJtants 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 the most distinct domestic varieties are, with rare exceptions, perfectly fertile when crossed, a~d 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. |