The average or mean heights were calculated in the ordinary rough method by adding up the measurements of all, and dividing the product by the number of plants measured; the result being here given in inches and decimals. As the different species grow to various heights, I have always for the sake of easy comparison given in addition the average height of the crossed plants of each species taken as 100, and have calculated the average height of the self-fertilised plant in relation to this standard. With respect to the crowded plants raised from the seeds remaining after the pairs had been planted, and of which only some of the tallest on each side were measured, I have not thought it worth while to complicate the results by giving separate averages for them and for the pairs, but have added up all their heights, and thus obtained a single average.

I long doubted whether it was worth while to give the measurements of each separate plant, but have decided to do so, in order that it may be seen that the superiority of the crossed plants over the self-fertilised, does not commonly depend on the presence of two or three extra fine plants on the one side, or of a few very poor plants on the other side. Although several observers have insisted in general terms on the offspring from intercrossed varieties being superior to either parent-form, no precise measurements have been given (1/8. A summary of these statements, with references, may be found in my 'Variation of Animals and Plants under Domestication' chapter 17 2nd edition 1875 volume 2 page 109.); and I have met with no observations on the effects of crossing and self-fertilising the individuals of the same variety. Moreover, experiments of this kind require so much time--mine having been continued during eleven years--that they are not likely soon to be repeated.

As only a moderate number of crossed and self-fertilised plants were measured, it was of great importance to me to learn how far the averages were trustworthy. I therefore asked Mr. Galton, who has had much experience in statistical researches, to examine some of my tables of measurements, seven in number, namely, those of Ipomoea, Digitalis, Reseda lutea, Viola, Limnanthes, Petunia, and Zea. I may premise that if we took by chance a dozen or score of men belonging to two nations and measured them, it would I presume be very rash to form any judgment from such small numbers on their average heights. But the case is somewhat different with my crossed and self-fertilised plants, as they were of exactly the same age, were subjected from first to last to the same conditions, and were descended from the same parents. When only from two to six pairs of plants were measured, the results are manifestly of little or no value, except in so far as they confirm and are confirmed by experiments made on a larger scale with other species. I will now give the report on the seven tables of measurements, which Mr. Galton has had the great kindness to draw up for me.

["I have examined the measurements of the plants with care, and by many statistical methods, to find out how far the means of the several sets represent constant realities, such as would come out the same so long as the general conditions of growth remained unaltered. The principal methods that were adopted are easily explained by selecting one of the shorter series of plants, say of Zea mays, for an example."

TABLE 1/1. Zea mays (young plants). (Mr. Galton.)

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed, as recorded by Mr. Darwin.

Column 3: Self-fertilised, as recorded by Mr. Darwin.

Column 4: Crossed, in Separate Pots, arranged in order of magnitude.

Column 5: Self-fertilised, in Separate Pots, arranged in order of magnitude.

Column 6: Crossed, in a Single Series, arranged in order of magnitude.

Column 7: Self-fertilised, in a Single Series, arranged in order of magnitude.

Column 8: Difference, in a Single Series, arranged in order of magnitude.

Pot 1 : 23 4/8 : 17 3/8 :: 23 4/8 : 20 3/8 :: 23 4/8 : 20 3/8 : -3 1/8. Pot 1 : 12 : 20 3/8 :: 21 : 20 :: 23 2/8 : 20 : -3 2/8. Pot 1 : 21 : 20 :: 12 : 17 3/8 :: 23 : 20 : -3. Pot 1 : - : - :: - : - :: 22 1/8 : 18 5/8 : -3 4/8. Pot 1 : 22 : 20 :: 22 : 20 :: 22 1/8 : 18 5/8 : -3 4/8.

The Effects of Cross and Self-Fertilisation in the Vegetable Kingdom Page 10

19th Century English Literature

Charles Darwin

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