In comparing the two sets, the eye alone was never trusted.  Generally the height of every plant on both sides was carefully measured, often more than once, namely, whilst young, sometimes again when older, and finally when fully or almost fully grown.  But in some cases, which are always specified, owing to the want of time, only one or two of the tallest plants on each side were measured.  This plan, which is not a good one, was never followed (except with the crowded plants raised from the seeds remaining after the pairs had been planted) unless the tallest plants on each side seemed fairly to represent the average difference between those on both sides.  It has, however, some great advantages, as sickly or accidentally injured plants, or the offspring of ill-ripened seeds, are thus eliminated.  When the tallest plants alone on each side were measured, their average height of course exceeds that of all the plants on the same side taken together.  But in the case of the much crowded plants raised from the remaining seeds, the average height of the tallest plants was less than that of the plants in pairs, owing to the unfavourable conditions to which they were subjected from being greatly crowded.  For our purpose, however, of the comparison of the crossed and self-fertilised plants, their absolute height signifies little.

As the plants were measured by an ordinary English standard divided into inches and eighths of an inch, I have not thought it worth while to change the fractions into decimals.  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.