Nature of Evolutionary Progress 55 



steers successfully its course through this difficult 

 world — life tries dropping out or damaging or alter- 

 ing at hazard one part after another. This is a proc- 

 ess that has been studied on a vast scale in certain 

 organisms ; notably in the famous fruit fly on which, 

 in such large degree, our knowledge of heredity and 

 variation is based. Life produces types that have no 

 eyes, and that therefore have no chance for continued 

 existence. It produces types that have no wings or 

 have wings that are useless for flying ; types with too 

 many legs that get in each other's way; types with 

 imperfect sense organs; types that are monstrous, 

 types that are weak, types whose parts are unco- 

 ordinated in their action ; types that carry within 

 themselves the seeds of their own destruction. In 

 hundreds of ways, life produces imperfect types, 

 many that cannot continue to exist even under the 

 best of conditions; many others that, under most 

 favorable conditions, weakly carry on for a few 

 generations but die under the first change of fortune. 

 These are not matters of theory; hundreds of such 

 imperfect and inefficient types, even in single species, 

 have been studied in detail, described in full, their 

 origin and fortune and fate followed with thorough- 

 ness.^ Indeed, when one studies minutely and in detail 

 for generation after generation the reproduction of 

 any organism, including in his view large numbers of 



1 For descriptions of great numbers of these imperfect types in 

 a particular organism, see Morgan, Bridges and Sturtevant, The 

 Genetics of Drosophila, particularly the list of mutant types at the 

 end. See also, for full details, the papers referred to in the bibliog- 

 raphy of the above work. 



