138 GENETIC VARIATIONS 



If this is the situation, where are we left as to the source of 

 the variations in genie materials that must underlie evolution 

 in higher organisms? 



Some investigators have expressed the opinion that such 

 variation is not required; that mere change of combination 

 and grouping, without alteration in the nature of any gene, 

 can in some way give rise to progressive evolution; can have 

 produced the seeming differentiation among genes now 

 existing. To most, however, this appears unintelligible. 



Possibly a type of gene change that is in the nature of 

 development rather than in that of disintegration and abnor- 

 mality will eventually be detected. When this is demon- 

 strated, the road for progressive evolution will be open ; until 

 this is demonstrated genetic science appears to be left with 

 the task that Osborn has called The Search for the Unknown 

 Factor in Evolution. 



