50 EVOLUTION AND GENETICS 



brids, the mule for example, it has been shown that 

 the maternal and paternal chromosomes fail to pass 

 successfully through this ordeal with the result that 

 later they are separated very irregularly. In conse- 

 quence the germ-cells contain all kinds of assort- 

 ments of the chromosomes and become abnormal. 

 The result is that the individual is sterile. 



While these observations do not explain why the 

 chromosomes fail to unite, thev do account for the 

 sterility of the hybrid. Until we learn more concern- 

 ing the conditions that bring about the union of the 

 chromosomes, it may be unsafe to offer any expla- 

 nation of the process ; nevertheless, for the present 

 at least, it is not irrational to ascribe the failure to 

 the differences in the hereditary factors carried by 

 the chromosomes of the two species. 



Now while all this will, I think, be conceded as 

 theoretically possible, the fact remains that in no 

 case has a mutant type been seen to arise that has 

 produced individuals that are fertile inter se, but 

 sterile with the parent species.^ Bateson has recently 

 emphasized this point and has insisted that until it 

 can be met we are not justified in assuming that new 

 species are formed by mutation. 



He says: "The production of an indubitably ster- 

 ile hybrid from completely fertile parents which 

 have arisen under critical observation from a com- 



1 Plough has recently reported a case that comes very near to fulfilling 

 these conditions. 



