WHITE-OCELLI. 235 



the character had persisted in undiminished frequency from June, 

 1912, to May, 1919, a period that represented fully 175 genera- 

 tions of flies. During this period the black stock had been carried 

 on in mass-cultures. Every two weeks a new culture was started 

 by transferring, without examination or selection, enough flies 

 to insure breeding. In such mass-cultures overcrowding is ex- 

 treme, and, in spite of. the great numbers of parents, not many 

 more offspring succeed in hatching than hatch from successful 

 pair-cultures. The competition grows keener with the age of 

 the culture, since the number of larvae is continually increasing 

 from eggs laid each day, while the quantity of available food soon 

 begins to diminish and its quality becomes progressively poorer. 

 The mass-culture method of breeding thus exercises a strong and 

 continuous selection against the perpetuation of the weaker or 

 slower hatching individuals or types. In several instances mixed 

 stocks have been started with equal numbers of different muta- 

 tions, and this stock transferred without selection through several 

 generations. Watch was kept, and in these cases there has been 

 a progressive change in the composition of the stock, rapid at 

 first, until the numbers of one type were quite small, and there- 

 after slower but in the same direction. Recessive characters of 

 very low viability may persist for many generations as a small 

 proportion of the population. Their existence is maintained by 

 the inter-crossing of the heterozygotes, whereby the mutant gene 

 escapes the adverse selection that the mutant character suffers. 

 Certain of our mutations are so sensitive to larval overcrowding 

 that the ratios in mass-cultures and in pair-cultures seem to 

 belong to different systems of heredity. Thus, the character 

 strap approaches I in 4 in pair-cultures, but may approximate 

 I in 1 6 in sister mass-cultures. 



The persistence of the white-ocelli character in undiminished 

 proportion through 175 generations of forced competition means 

 that the mutant is under no disadvantage. Such a mutant might 

 easily survive in nature, and one slightly advantageous might 

 ultimately supplant the original type. 



