190 Degeneration in the Ostrich 



of the stock ; not a mere result of the recombination of Mendelian factors. 

 This is the general and fundamental point at issue. 



"2. More specifically, he holds it to be an actual change in a single 

 unit factor; this single factor changes its grade in a continuous and 

 quantitative manner. 



"On the other side, the critics of these views maintain that the 

 changes shown are not actual alterations in the hereditary constitution 

 at all, but are mere results of recombinations of Mendelian factors. And 

 specifically, they find a complete explanation of such results as those of 

 Castle in the hypothesis of multiple modifying factors." 



Prof Jennings then proceeds to discuss in much detail recent work 

 on colour changes in the eyes of Drosophila, most of which are minute 

 differences. These are claimed by the mutationists to be due to the 

 influence of multiple modifying factors, by means of which ■ a visible 

 character may be modified in the finest gradations by alterations in 

 diverse parts of the germinal apparatus, and by multiple allelomorphs 

 which show that a single unit factor may exist in a great number 

 of grades. " The very facts known for Drosophila show that there is 

 nothing to prevent a passage from one extreme to the other by minute 

 changes, just as is held by the palaeontologists and selectionists, although 

 change by large steps occurs also "..." the mutationist thinks of all these 

 numerous grades as after all essentially discontinuous, as a series of steps 

 so minute that the difference between one and the next one is not 

 detectible. His opponent, on the other hand, perhaps thinks of the 

 series as actually continuous. But the difference is not a pragmatical 

 one ; when steps become so minute as to be beyond detection, the ques- 

 tion whether they exist becomes metaphysical." 



Until we have direct evidence to the contrary the same principles 

 must be held with regard to the smallest step in the degenerative process 

 as to the dropping out of the large individual characters, which we 

 usually think of as mutations. They are all changes held to be effected 

 during the mitotic reconstruction of the zygote, and Morgan has shown 

 that in Drosophila each change almost certainly involves a change in 

 a definitely localized part of a chromosome ; and with good reason he 

 regards each step, whatever its size, as a unit character or mutation. 

 Though to de Vries a mutation, and to Bateson a discontinuous variation 

 represents usually a large, obvious character, closer analysis shows no 

 reason why the somatic expression of every factorial change, however 

 small its degree, should not be regarded in the same sense. In this 

 case every stage in the diminution of the claw or of a plume would 



