284 JENNINGS! CHANGES IN HEREDITARY CHARACTERS 



color or form in plants; the dropping out of definite Mendelian 

 units in Drosophila and elsewhere ; the transformation of particu- 

 lar Mendelian units into some other condition. 



So much then may serve as an outline of a prevailing theory; 

 organisms forming a multitude of diverse strains with diverse 

 genotypes; the genotype a mosaic of parts that are recombined 

 in Mendelian inheritance; selection a mere process of isolating 

 and recombining what already exists; large changes occurring 

 at rare intervals, through the dropping out of bits of the mosaic, 

 or through their complete chemical transformation; evolution 

 by saltations. 



Certain serious difficulties appear in this view of the matter; 

 I shall mention merely two of them, for their practical results. 

 One is the very existence of the minutely differing strains, 

 which forms one of the main foundations for the genotype theory. 

 How have these arisen? Not by large steps, not by saltations, 

 for the differences between the strains go down to the very 

 limits of detectibility. On the saltation theory, Jordan's view 

 that these things were created separate at the beginning seems 

 the only solution. 



Secondly, to many minds there appears to be an equally great 

 difficulty in the orig'n by saltation of complex adaptive structures, 

 such as the eye. I shall not analyze this difficulty, but merely 

 point to it and to the first one mentioned, as having had the 

 practical effect of keeping many investigators persistently at 

 work looking for something besides saltations as a basis for 

 evolution; looking for hereditary changes that would permit a 

 continuity in transformation. Some have been searching in the 

 complex phenomena of biparental inheritance; here Castle is 

 to be first named, and in a later lecture you will hear of the views 

 to which he has been led. Others, like Prof. H. F. Osborn, have 

 been searching from this point of view the paleontological records. 

 Others of us have taken up the problem in uniparental repro- 

 duction; it is here that my own work falls, and of this I will for 

 a moment speak. 



Where reproduction is from a single parent we meet the 

 problem of inheritance and variation in its simplest form; for 



