356 



EVOLUTION AND GENETICS 



aphid constitute a clone. Such animals as the Protozoa and 

 Hydra also undergo a series of asexual divisions which give rise 

 to clones before sexual reproduction intervenes to luring about a 

 reassortment of characters. 



Homozygous crosses are not only uncommon in nature but also 

 difficult to obtain in the laboratory. With respect to single char- 



w 



V 



1, 



1/1 



7 H- 



00 



CJ 



Fig. 197. — Typical heads from seven pure lines of Defiance wheat. (From 

 Babcock and Clausen's Genetics in Relation to Agriculture. McGraw-Hill 

 Book Company, Inc., by permission.) 



acters this difficulty is not encountered and lines homozygous for 

 one or a few characters have been secured many times and in 

 many species. 



Selection in Pure Lines. When Johannsen had isolated his 

 nineteen pure lines of beans he found that no matter what the 

 size of the parent seed, those which it produced fluctuated about 

 the mean for the pure line to which it belonged. Walter has 

 graphically indicated these results in a diagram which is repro- 

 duced in Figure 198. In all lands of pure lines the effects of 

 selection have been hkewise negative. Aphids, daphnids, Droso- 



