STRAIN AND INDIVIDUAL IDIOSYNCRASIES 35 



sistence and final definite establishment of these larval characteristics 

 must have been due to a potency in inheritance at least equivalent to 

 that shown by such discontinuous variations as De Vries's mutations. 



There is an important significance then, to my mind, in this differ- 

 ence of conditions between the cocoon characteristics and the larval 

 characteristics of the silkworm. On the one hand we have different 

 characteristics appearing originally, in most cases at least, as slight 

 fluctuating or Darwinian variations, selected, fostered and fixed by the 

 careful attention and manipulation of the breeder and by these means 

 finally elevated to a condition apparently stable and of value equivalent 

 to that of the usual differences in natural races or species. On the 

 other hand we have, in the larval characteristics, a series of differences 

 or variations which are strictly natural in their establishment. This 

 establishment however cannot have come about by selection; not by 

 natural selection, because during the many generations in the course of 

 which this establishment has been brought about, the silkworm has 

 not been exposed to natural selection ; not by artificial selection, prob- 

 ably, because the characteristics are of no interest to the breeders. The 

 establishment has come about, then, through natural methods, probably 

 by the appearance of sudden discontinuous variations or mutations, 

 which have been sufficiently potent in inheritance to have maintained 

 themselves. 



Despite this difference in the method of establishment the two 

 sets of characteristics appear now on their faces to be of equivalent 

 character and worth. But an experimental study of them by a pro- 

 tracted series of matings, pure and cross, shows that they are not of 

 equivalent worth. The larval characteristics, established by Nature, 

 are unbreakable, behave consistently and rigorously in inheritance 

 through all possible manipulation. The cocoon characteristics, estab- 

 lished artificially, break down under manipulation, are inconsistent in 

 their inheritance behavior and reveal an instability which distinguishes 

 them clearly and importantly from the larval characteristics. 



And yet there are important and suggestive points of likeness. 

 The cocoon characteristics as they stand today are discontinuous in 

 their nature and show a strong tendency to become fixed, stable and 

 consistent in inheritance, this stability and consistency being exactly 

 of the type shown by the larval characteristics. In many crossings the 

 cocoon characteristics are inherited in purely alternative manner and 

 with close approximation to Mendelian proportions. In other cross- 

 ings, using the same characteristics in different strains or races, or, 



