58 PHYSIOLOGICAL GENETICS 



appeared in Mohr's work. Above the pictures the constitution 

 is marked, and below the pictures the percentage of individuals 

 showing the phenotype (percentage of penetrance). We realize 

 that the normal wing of the wild type ( + "'/ + ''' , ) occurs also in 

 the heterozygotes between wild and notched (vg no ), wild and 

 nicked (vg"'), and wild and vestigial (vg) and in the homozygous 

 nicked fly. The compound nicked-notched shows that 0.2 per 

 cent flies with a nick and 100 per cent scalloped flies are obtained 

 only in the compound vestigial-notched; all further data may be 

 read from the figure. 



Here, then, a series of alleles (into which, also, the heterozygotes 

 and compounds fit) produce a perfectly orderly series of effects, 

 with a simple quantitative arrangement of the phenotypes 

 resulting. It begins with no visible effect and leads through the 

 steps of a small effect in few, in many, in all individuals to 

 increased and still more increased effect up to the maximum that 

 is physiologically possible, and this maximum is at the threshold 

 of lethality. 



The study by Goldschmidt (1935c) of the development of these 

 quantitatively different grades of scalloping of the wings showed, 

 as reported above, that development is normal up to a certain 

 moment, when a degeneration of the tissue of the wing margin 

 sets in and proceeds up to a time in development when no 

 further destruction of wing area seems to be possible, and from 

 then on the rest of the wing finishes its normal development. It 

 was further show r n that, so far as the evidence goes, the earlier 

 and earlier onset of the degenerative process produces a greater 

 and greater amount of wing destruction. Thus the different 

 genetic constitutions are linked with a process that contains a 

 definite time element. Taking into account further the pheno- 

 copic effect (see page 9) of the same type produced if the 

 temperature shock or X-ray shock is applied at the proper time, 

 (as well as the facts to be reported later on a parallel shift of the 

 phenotype by dominance modifying genes), we may form a 

 picture of the whole process in terms of rates. At the present 

 juncture in the analysis of the case, this picture might be differ- 

 ently conceived in its details; but in its general form it must be 

 of the following type: The process of degeneration of the wing 

 tissue which results in scalloping may be due to the presence of 

 something that prevents — physically (permeability, viscosity) or 



