Other Kinds of Hybridizing 171 



a large number of steps, and the results of hybridization of such 

 forms may differ from the results of hybridization of single steps. 

 If new elementary species arise by single steps and are subjected 

 to crossing with the parent type, as would almost inevitably take 

 place in many cases, the problem of paramount importance, 

 from the point of view of evolution, is to study the results of such 

 hybridization. Crosses between already established species are 

 infrequent in nature and seem only in rare cases to give rise to 

 new species, hence their study is of secondary importance from 

 the standpoint of evolution, provided always that evolution 

 has taken place discontinuously. On the other hand, it is un- 

 fortunate in many ways to attempt to estimate the value of the 

 study of unit characters by the supposed bearing of the results 

 on the theory of evolution ; for while those who believe that evolu- 

 tion has taken place by simple discontinuous steps will ascribe 

 the highest value to studies of this kind, those who believe that 

 evolution has taken place in some other way will be likely to 

 underrate the importance of the results as a contribution to the 

 theory of heredity. Already the bitter controversies that the 

 publication of these results has aroused are less concerned with 

 the results themselves, that are accepted by all, than with the 

 imagined bearing of the results on the theory of evolution. The 

 intolerance that each side sees in the other is due not to the ac- 

 ceptance or denial of the results of the experimental work, but to 

 the arguments that pretend to show that evolution has or has 

 not taken place by discontinuous variation. Meanwhile there is 

 danger that we forget the importance of the experimental results 

 as a contribution to the study of heredity, whatever their bearing 

 on the theory of organic evolution may be. 



LITERATURE, CHAPTER X 



ACKERMANN. Tierbastarde. Kassel. 1898. 



BATESON, W. Materials for the Study of Variation. 1894. 



BATESON, W., and GREGORY, R. P. On the Inheritance of Heterostylism 



in Primula. Proceed. Roy. Soc. of London, LXXVI. 1905. 

 CONKLIN, E. G. The Cause of Inverse Symmetry. Anat. Anz. XXIII. 



1903. 



