34 • INHERITANCE IN SILKWORMS, I 



The condition is different in the case of the larval characters. 

 Here the inheritance behavior is consistent, is rigid. It can be prophe- 

 sied. It follows the Mendelian principles of alternative inheritance 

 with great fidelity. 



What is the reason for this difference between the inheritance 

 behavior of the larval characters and that of the cocoon characters? 

 What is the significance of this difference? 



In the five thousand years or less during which the mulberry silk- 

 worm has been the subject of man's ameliorating attention the principal 

 aim of all the manipulation by the various processes involved in arti- 

 ficial selection has been the modification of the cocoon characteristics. 

 The attempt has been to produce more silk, better silk, silk of one 

 color, silk of another color. As regards larval and imaginal char- 

 acters, much less attention and manipulation have been given. Docile, 

 disease-resistant and hearty-feeding larvae, prolific and sedentary moths 

 have been encouraged by selection. But larval patterns, diverse and 

 distinct though they appear to us today, have not been the product of 

 the breeder's work except as they may be correlated with valuable 

 cocoon characters and thus preserved by the way. The diversity in 

 larval pattern is a natural diversity ; the differences have appeared and 

 have persisted according to natural processes. 



Not so with the cocoon characters. Or at least only in so far as 

 natural variation has coincided with the breeder's wishes. The cocoon 

 colors have originated as fluctuating variations fostered, accumulated, 

 and fixed by careful, rigorous selection. Or if any of them have ap- 

 peared as discontinuous variations or sports they have been given from 

 the start all the advantage of the breeder's selective attention. 



But the larval patterns have had to make their way alone. How 

 have they come to exist then? As fluctuating variations fostered and 

 fixed by selection? No; for neither artificial selection (except in rare 

 possible cases of coincidence with a desirable cocoon variation), nor 

 natural selection have played any part in their history in the last 4000 

 or 5000 years. Then they have probably arisen as discontinuous 

 variations or sports, or as mutations, if the mutationists will admit 

 them to their charmed circle. But in order to persist, these discon- 

 tinuous larval variations or sports must have been endowed with a 

 certain potency or prepotency, which prevented them from being lost 

 or extinguished by interbreeding. If these discontinuous variations, 

 sports, or mutations, have arisen, as seems probable from the analogy 

 with other discontinuous variations, in small numbers, then the per- 



