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Show 280 SUMMARY OF MEASUREMENTS. C HAP. VII. by above five per cent. in height, or are inferior to them, we Inay conclude that mueh tho greater number of the ca es do not form real xceptions to the rule, · · - that a cross between two plants, unless those have been self-fertilised and exposed to nearly tho same conclitio,ns for Inany generations, gives a great ad vantage of so1ne kind to the offspring. Of the twenty-six cases, at least two, namely: those of Adonis and Bartonia, n1ay be wholly excluded, as the trials were worthle s from the extreme unhealthiness of tho })lants. In twelve other cases (thro trjals with Eschscholtzia hero included) the crossed plants either were superior in height to the self-fertilised in all tho other generations exceptino· the one in guostion, or they showed their superiority in some different n1annor, as in weig·ht, fertility, or in flow rin()' first· or ao·ain the b ' b ' cross-fertilis c1 flowers on the mother-plant wore much more productive of seed than tho self-fertilised. Deuucting these fourteen cases, thoro rmnain twelve in which tho crossed plants show no well-marked advantage over the self-fertilised. On the other hand, we have seen that there arc fifty-seven cases in which the crossed plants exceed the self-f rtilised in height b! at least five per cent., and generally in a much higher degree. But oven in the twelve cases just referred to, the want of any advantage on the crossed side is far from certain: with Thunborgia tho parentplants were in an odd semi -sterile con eli tion, and the offspring grew very unequally; with :Hibiscus and Apium Inuch too few plants were raised for the measurements to be trusted, and the cross-fertilised flowers of Hibiscus produced rather more sood than did tho selffertilised ; wit~ v andollia the crossed plants wore a little taller and heavier than the self-fertilised, but as they were less fertile the case must be left doubtful. CHAP. VII. TABLE A. 281 Lastly, with Pisum, Primula, the thre.e generations of Canna, and the throe of Nicotiana (which tog ther complete the twelve caRes), a cross between two plants certainly did no good or very little good to the offspring; but we have reason to believe that this is the result of these plants having- boon self-fertilised and cultivated under nearly uniform conditions for several generations. The same result followed with the expe~iInental plants of Ipomcca find Mi1nulus, and to a certain extent ~ith some other species, which had been intentionally treated by me in this manner; yet we know that these species in their nonnal condition profit greatly by be in 0' in tercrossed. There is, therefore, not a single b . case in Table A which affords decisive evidence against the rule that a cross between plants, the progenitors of which have been subjected to somewhat diversified conditions, is beneficial to the offspring. This is a surprising conclusion, for from the analogy of domesticated animals it could not have been anticipated, that the good effects of crossing or the evil effects of selffertilisation would have been perceptible until the plants had been thus treated for several generations. ~:rhe results given in Table A may be looked at under another point of view. Hitherto each generation has been considered as a separate case, of which there are eighty-three ; and this no doubt is the more correct method of comparing the crossed and selffertilised plants. But in those cases in which plants of the sa1ne species were observed during several generations, a general average of their heights in all the generations together may be m.ade; and such averages are given in Table A; for instance, under Ipomma the general average for the plants of all ten gener~t~ons is as 100 for the crossed, to 77 for the solf-fertihsocl |