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THE AMERICAN NATURALIST [Vol. XLIX 



/. Preliminary Outline 



1. Introduction 



Comparative studies along statistical lines of the results 

 produced by cross breeding and close breeding afford 

 data of value bearing on the problem of evolution as well 

 as the subsidiary problem of the origin of amphimixis. 

 It has long been assumed ( Weismann, 76) that sev p\-i=?ted 

 primarily to increase variability and with the further as- 

 sumption that the variations thus produced were heritable 

 and accumulated, the differentiation of organisms was 

 logically explained. As a corollary to such a conclusion 

 the belief has long been prevalent that the offspring of 

 organisms produced by cross breeding were as a group 

 more variable than those produced by close breeding, an 

 idea which gained further acceptance in connection with 

 the investigations of Castle ('06), Jennings ('08, '09, '12, 

 '13) and others interested in problems of genetics. That 

 there was excellent evidence for exactly an opposite view 

 and that an analysis of the results presented by the in- 

 vestigators mentioned above did not bear out the conclu- 

 sion that variability was increased by cross breeding has 

 been pointed out by the writer (Walton, '08, '12, '14) in 

 some earlier papers. 



The importance of arriving at a correct conclusion con- 

 cerning the part played by hybridization and cross breed- 

 ing in evolution can not be overestimated. If units are 

 merely redistributed and form characters resulting in no 

 actual evolutionary progress, work along Mendelian lines 

 tends rather to obscure the facts of value toward solving 

 the problem of the origin of species as well as that of evo- 

 lutionary control in animal and plant breeding. It is 

 therefore well to obtain data from as many sources as 

 possible bearing on the question. 



Among the species of Spirogyra, a group of algae be- 

 longing to the class Conjugatae, there are several which 

 reproduce both by lateral conjugation (Fig. 1, A) where 



