( Ixxxviii ) 



as continuous as can be detected." Variation occurred in 

 many diverse characters and gave rise to many diverse com- 

 binations of characters. " Any set of characters might vary 

 independently of the rest. The hereditary variations which 

 arose were of just such a nature as to produce from a single 

 strain the hereditarily differentstrains that are found in nature." 

 After giving an account of these results obtained from an 

 organism with uniparental reproduction, the author briefly 

 refers to Osborn's palaeontological evidence " for evolution 

 by minute continuous variations which follow a single definite 

 trend," and to Castle's evidence that " in rats he can, by 

 selection, gradually increase or decrease the amount of color 

 in the coat, passing by continuous stages from one extreme 

 to the other," the change being " an actual change in the 

 hereditary characteristics of the stock ; and not a mere result 

 of the recombination of Mendelian factors." From this 

 point the author proceeds to a critical examination of results 

 obtained by T. H. Morgan and his associates upon a species 

 which comes directly within the survey of the Entomological 

 Society, viz. the fruit-fly Drosophila. The species has normally 

 a red eye, but in the few years during which these researches 

 have been conducted it has come to present " seven grada- 

 tions of color between white and red, each gradation heritable 

 in the normal Mendelian manner " — " red, blood, cherry, 

 eosin, buff, tinged, white." " Three of these grades have 

 been discovered in the last five months. It would not require 

 a bold prophet to predict that as the years pass we shall 

 come to know more of these gradations, till all detectible 

 differences of shade have been distinguished, and each shown 

 to be inherited as a Mendelian unit. Considering that the 

 work on Drosophila has been going on only about seven or 

 eight years, this is remarkable progress toward a demon- 

 stration that a single unit factor can present as many grades 

 as can be distinguished. . . ." But this is not all. As regards 

 the middle member of the series of eye colours, eosin, Bridges 

 lias found seven modifying factors, each of which alters its 

 intensity and gives rise to a secondary grade of colour. Now 

 each of these modifying factors are described " specifically as 

 mutations; as actual changes in the hereditary material." 



