246 PLANT HYBRIDIZATION BEFORE MENDEL 



of a usual kind, and in which no great varieties of nature were to be 

 found. Natural selection must have had little influence on the charac- 

 teristics that were to be examined. These must be measurable, variable, 

 and fairly constant in the same individual. The result of numerous in- 

 quiries, made of the most competent persons, was that I began my ex- 

 periments many years ago on the seeds of sweet peas. . . ." (p. 71.) 



At first both size and weight were determined but, after becom- 

 ing assured of the equivalence of the two methods, Galton con- 

 fined himself to the weights, in that they were more easily ascer- 

 tained than the measurements. 



"It is more than 10 years (from 1889) since I procured these data. 

 They were the result of an extensive series of experiments on the pro- 

 duce of seeds of different sizes, but of the same species, conducted for 

 the following reasons. I had endeavoured to find a population possessed 

 of some measurable characteristic that was suitable for investigating the 

 causes of the statistical similarity between successive generations of a 

 people." (p, 80.) 



As to the selection of sweet peas, Galton says : 



"They do not cross-fertilize, which is a very exceptional condition 

 among plants ; they are hardy, prolific, of a convenient size to handle, 

 and nearly spherical ; their weight does not alter perceptibly when the 

 air changes from damp to dry, and the little pea at the end of the pod, 

 so characteristic of ordinary peas, is absent in sweet peas." (p. 80.) 



Seven sets were selected for planting, containing ten seeds 

 each, graduating in weight from the heaviest to the lightest. 



After speaking of the immense amount of labor involved in 

 the details of the experiment, Galton says : 



"The results were most satisfactory. They gave me two data, which 

 were all that I wanted in order to understand, in its simplest approxi- 

 mate form, the way in which one generation of a people is descended 

 from a previous one ; and thus I got at the heart of the problem at 

 once." (p. 82.) 



The tabulated results of this work upon the weights of seeds in 

 two succeeding generations of sweet peas were such as to demon- 

 strate what Galton called the fact of filial regression. 



"It will be seen," he says, "that for each increase of one unit on the 

 part of the parent seed, there is a mean increase of only one-third of a 

 unit in the filial seed ; and again that the mean filial seed resembles the 

 parental when the latter is about 15.5 hundredths of an inch in diameter. 

 Taking 15.5 as the point towards which Filial Regression points, what- 

 ever may be the parental deviation from that point, the mean Filial 

 Deviation will be in the same direction, but only one-third as much." 

 (p. 225.) 



In the investigation of the inheritance of human stature. Gal- 



