‘ Other Kinds of fTybridizing 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 Grecory, R. P. On the Inheritance of Heterostylism 
in Primula. Proceed. Roy. Soc. of London, LXXVI. 1905. 
Conxiin, E. G. The Cause of Inverse Symmetry. Anat. Anz. XXIII. 
1903. 
