( Ixxxvii ) 



The author points out the difficulty involved in the very 

 existence of the minute strains, which must surely have arisen 

 at some time or other, and arisen " not by saltations, for the 

 differences between the strains go down to the very limits 

 of detectibility." Then there is the difficulty of " complex 

 adaptive structures, such as the eye." 



Therefore many investigators could not feel satisfied with 

 the favourite theory and have been " looking for something 

 besides saltations as a basis for evolution; looking for here- 

 ditary changes that would permit a continuity in transforma- 

 tion." Among these investigators W. E. Castle has been 

 searching in the phenomena of biparental inheritance, H. F. 

 Osborn in the records of palaeontology, and the author in 

 organisms subject to uniparental reproduction. In this latter 

 " we meet the problem of inheritance and variation in its 

 simplest form ; for there is nothing which complicates genetic 

 problems so enormously as does the continual mixing of 

 diverse stocks in biparental inheritance. In uniparental re- 

 production we have but one genotype to deal with; we can 

 be certain that no hereditary characters are introduced from 

 outside that genotype." 



Accepting as a foundation the facts already mentioned " as 

 to the make-up of the species out of a great number of diverse 

 stocks; as to the usual efiects of selection being nothing 

 save the isolation of such pre-existing stocks," the author 

 undertook " a most extensive and intensive study of heredity, 

 of variation, and of the effects of selection for long periods " 

 within a single stock of a favourable organism. This he 

 found in the Rhizopod Diffiugia corona, which has " numerous 

 distinctive characters, all congenital; all inherited in a high 

 degree ; yet varying from parent to offspring also ; none of 

 these characters changed by growth or environmental action 

 during the life of the individual." 



" Long-continued work showed that a single strain of this 

 animal, all derived by fission from a single parent, does 

 differentiate gradually, with the passage of generations, into 

 many hereditarily diverse strains." While these variations 

 " arose in some few cases by rather large steps, or ' saltations,' " 

 " the immense majority were minute gradations. Variation is 



