80 NATURAL INHERITANCE. [chap, 



that I need specify were the very first that I used ; they 

 refer to the sizes of seeds, which are equivalent to the 

 Statures of seeds. I both measured and weighed them, 

 but after assuring myself of the equivalence of the 

 two methods (see Appendix C), confined myself to 

 ascertaining the weights, as they were much more 

 easily ascertained than the measures. It is more 

 than 10 years since I procured these data. They 

 were the result of an extensive series of experiments 

 on the produce 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, as will here- 

 after be discussed in Chapter VIII. At last I determined 

 to experiment on seeds, and after much inquiry of very 

 competent advisers, selected sweet-peas for the purpose. 

 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. I began by weighing thousands of them individ- 

 ually, and treating them as a census officer would treat 

 a large population. Then I selected with great pains 

 several sets for planting. Each set contained seven 

 little packets, numbered K, L, M, N, 0, P, and Q, 

 each of the seven packets contained ten seeds of almost 



