OCR Text |
Show 18 INTRODUCTORY REMARKS. CIIAP. I. that I. n th e seven 1n ' nds of plants' w. hose measurem, ontR I h1a ve examm. o d , the rat I'O between the height.s o.f tho crosf.: i,c d al.l ( . of t h e se lf- 1.c or t'Il I'S od r"(~n Oo' es in five cases w1tlun vorv• narro.w limits. In Zea mays it is as 100 to 84, and in tho oth r 1t ranges between 100 to 76 and 100 to 86." . H The determination of the variability (measured by what 1s technically called the 'probable error') is a problem of m?ro delicacy than that of determining the moa~R, and I d?ubt, after making many trials, whether it is po. s1blo to donvo uRefnl conclusions from these few observation. . \Vo ouO'}lt to hn,vo measurements of at least fifty plants in each caRe, In or lor to be in a position to deduce fair ro. ults. One fa t, however, bearing on variability, is very evident~~ 1nost caseR, .thongh not in Zea mays, viz., that the self-fertilised l)~antR m ln.do tl1o larger number of exceptionally small specnnon. , wlnlo the crossed are more generally full grown." ''Those O'roups of cases in which moasurem nt. have been made of a few of the tallest plants that grow in row.·, each of which contained a multitude of plants, show v ry lcarly that the crossed plants exceed tho self-fortili. od in height, but they do not tell by inference anything about their re. ·p ctiYo mean values. If it should happen that a series is known to follow the law of error or any other law, and if tho nlnnber of individuals in the series is known, it would he always pos. iblo to reconstruct the whole series when a fragment of it has been given. But I fi~d no such method to be applicable in the present case. Tho doubt as to the number of plants in each row is of minor importance; the real difficulty lies in our ignorance of the precise law followed by tho series. Tho oxp rienco of the plants in pots does not help us to determine that Jaw, because the observations of such plants are too few to onnb]e us to lay down more ' than the middle terms of the . cries to which they belong with any sort of accuracy, whoroaR tho caReR we are now considering refer to one of its extremities. There are other special difficulties which need not be gone into, as the one already mentioned is a complete bar." Mr. Galton sent me at the same time graphical representations which he had made of the mcasure: rnents, and they evidently form fairly regular curves. He appends the words "very good" to those of Zea n.nd CHAP. I. INTRODUCTORY REMARKS. 19 Limnanthes. He also calculated the average height of the crossed and self-fertilised plants in the seven tables by a more correct method than that followed by me, namely, by including the heights, as estimated in accordance with statistical rules, of a few plants which died before they were measured; whereas I 1nerely added up the heights of the survivors, and uivided the sum by their number. The difference in our results is in one way highly satisfactory, for the average heights of the self-fertilised plants, as deduced by Mr. Galton, is less than mine in all the cases excepting one, in which our averages are the same ; and this shows that I have by no means .exaggerated the superiority of the crossed over the self-fertilised plants. After the heights of the erossed and self-fertilised plants had been taken, they were· sometimes cut down close to the ground, and an equal number of both weighed. This method of cornparison gives very striking results, and I wish that it had been oftener followed. Finally a record was often kept of any 1narked difference in the rate of germination of tht-~ crossed and self-fertilised seeds,-of the relative periods of flowering of tho plants raised from them,-ancl of their productiveness, that is, of the nu1nber of seedc. apsules which they produced and of the averag·e number of seeds which each capsule contained. When I began my experin1ents I did not intend to raise crossed and self-fertilised plants for more than a single generation; but as soon as the plants of the first generation ·wore in flower I thought that I would raise one more generation, and acted in the followin a- 1nanner. Several flowers on one or more of the self'"-"' fertilised plants were again self-fertilised ; and several c 2 |