The Process of Heredity C09 



A number of other differentiating characters might be listed 

 here, but it seems advisable to defer mention of many such charac- 

 ters until they can be treated in a connection more intelligible. 



Method 



Since the problem in hand has proven to be so largely one of 

 method, it seems necessary to preface the bare statement of the 

 method finally evolved with an historical account of some of the 

 steps in the evolution of this method. 



The first season's work brought out so much of contradiction 

 and ill success in rearing both pure and hybrid strains, that it 

 seemed necessary to become more familiar with the physiology 

 and behavior of the two species of fish used. This study was 

 carried on during the early part of the second season and resulted 

 in the discovery of many interesting facts about the spawning 

 behavior, the significance of the sexual dimorphism displayed, 

 and the sure signs, morphological and physiological, of high sexual 

 tone. Although a full account of these phenomena has been 

 published,^ it will be convenient in this place to set down some of 

 the facts that have a bearing on the present work. 



I Spawning Behavior and Sexual Dimorphism 



These two species of fish, like other fish, have a well-defined 

 breeding season. That of F. majalis is somewhat earlier than 

 that of F. heteroclitus and lasts for a shorter time. In both species 

 there is a still more restricted period during which spawning is 

 carried on most actively, and during which both sexes are at the 

 height of their sexual tone. This sexual climax comes earlier 

 in F. majalis than in F. heteroclitus, by about two weeks, and over- 

 laps the corresponding period of the latter by about three or four 

 weeks. 



During this period of a few weeks, which may be called the 

 spawning period proper, both species show marked changes in 

 structure and behavior. In the males an intensification of pig- 

 ment appears over the entire body and especially in certain 



^ H. H. Newman. Biol. Bull., vol. xii, no. 5, April, 1907. 



