Selection Among Diverse Strains 61 



were supposed to have. A great school of biologists, the 

 immediate followers of Darwin, forming the so called English 

 biometrical school, set themselves the problem of measuring 

 variation, inheritance, the effects of selection, and from 

 these the rate of evolution. In so doing they assumed these 

 existing differences as variations, and based their calcula- 

 tions upon these ; they found of course a high degree of in- 

 heritance from diverse parents, and they found that by 

 selection rapid progress could be made in changing the 

 species. But if we examine a species made up of a lot of 

 hereditarily diverse strains (for example, Diiflugia corona), 

 it is evidently easy by selection of a particular character to 

 obtain a set of animals that differ from the average in that 

 respect; one merely picks out representatives of the races 

 that have the character for which we are selecting. Thus, 

 in Difflugia corona, if one desires to increase the average 

 numbers of spines, he will pick out parents with many 

 spines. These belong to races in which a large number 

 of spines is hereditary, so that after selection the progeny 

 produced will have a higher number of spines than was the 

 average for the species before selection. By continuing 

 the process of selection, we gradually exclude more and more 

 completely the races with few spines, and so by selection we 

 make steady progress in increasing the number of spines in 

 this animal. Similar results follow from selecting for any 

 other inherited character; and in any species composed of 

 diverse races, the same sort of results are reached. 



Thus in a population of Paramecium we can easily obtain 

 by selection all sorts of apparent hereditary alterations in 

 size; in nutritive peculiarities; in resistance to heat or to 

 chemicals ; in reproduction ; in all sorts of characters. All 

 these and many other results have been produced again and 

 again, in this and in other species. But what we really do 



