378 castle: role of selection in evolution 



Jennings, selecting a new species of Protozoa, more favorable 

 for precise quantitative observation, also obtained a different 

 result. He now found that among the observed fluctuations in 

 size, those of a genetic character were included, so that by 

 repeated selection races could be produced which were progres- 

 sively larger or smaller, rougher or smoother. This is fully in 

 harmony with the observations of Stout who found that varia- 

 tions in Coleus arising in asexual propagation were capable of 

 further propagation. It also harmonizes with the observation 

 of Shamel as regards the occurrence in citrous fruits of bud 

 variations which are important enough to warrant propagation in 

 economic work ; and further, with Winkler's clear demonstration 

 of the occurrence in the tomato and the night-shade of gigas like 

 mutations, arising first in single somatic cells, which asexually 

 propagated produce entire plants of a new type which then are 

 self-perpetuating by seed. We also have the observations of 

 East that in the asexual propagation of the potato occasional 

 bud variations may occur which are similar in nature to unit- 

 character variations in reproduction by seed. It is accordingly 

 clear that the pure-line principle does not apply without excep- 

 tion to asexually reproducing organisms any more than it does 

 to self-fertilizing ones. It is true, however, that genetic varia- 

 tions are much less common among such organisms than among 

 those produced by cross-fertilization. Herein lies the justi- 

 fication of present agricultural practice in the breeding of self- 

 fertilized cereals, and of horticultural practice in the propagation 

 by grafts, runners, layers, etc., of superior individual plants. 



4. Attempts to extend the pure line principle to organisms 

 which are not self -fertilizing (and this includes all the domestic 

 animals and many cultivated plants) have met with small 

 success. Morgan indeed assumes that it applies to his races of 

 Drosophila up to a certain point, the point at which mutation 

 begins, but the mutations which he recognizes are so numerous, 

 so minute in many cases, and so fluctuating in others, that it 

 becomes a question whether his "mutations" are not just ordi- 

 nary heritable variations. Morgan would undoubtedly admit 

 this since he claims that all heritable variations arise as mutations, 



