284 DARWINISM TO-DAY. 



a time, it is easy to see that if some of these produce young 

 which do not proceed farther along the orthogenetic line, 

 that is, do not vary farther, while others produce progeny 

 that tend to move on along the line or lines of determinate 

 variation, new species can be dropped, as it were, out of th^ 

 general course of the orthogenetic evolution all along. An 

 if these persist we have a series of distinct organic forms all 

 related chainwise although living simultaneously and in the 

 same region. This cessation of development can lead to 

 manv added new forms when it occurs in the form of 



./ 



Heterepistase, that is, where only a few characteristics re- 

 main fixed at some early developmental stage while others 

 go on. By the fixing or cessation of development in differ- 

 ent small groups of characteristics, and in different combina- 

 tions of these groups many new species may result. All 

 cases of so-called atavism are interpreted by Eimer simply 

 as examples of his Entwicklungsstillstand 3 this cessation of 

 development occurring in the atavistic organs at a very early 

 stage. 



We should not omit mention, in connection with Eimer's 

 theory, of a point upon which he lays great stress, and that 

 is that his theory is not the result of pure speculation, but is 

 the unavoidable conclusion arrived at by long years of 

 specific observations and study of the facts obtaining in the 

 case of the relations, conditions, and course of evolution of 

 certain groups of organisms. Eimer made careful and ex- 

 tended studies of the wing-patterns of two large groups of 

 butterflies, and of certain lizards and birds, and it is on the 

 basis of these studies in particular that his theory was 

 formulated. It is certainly to be admitted that his exhaust- 

 ive and most suggestive account of the relations of species 

 and patterns in the swallow-tailed and certain other butter- 

 flies makes a very strong argument against the validity of 

 natural selection as an explanation of these conditions. And 

 the example of Eimer's prolonged and minute study of 



