446 THE AMERICAN NATURALIST [VOL. XLII 



cross of the type AbC X aBC = ABCab provided the 

 same relations exist among the several allelomorphs as 

 before. In this case the character C is latent by hypo- 

 stasis in both parents. This condition has been realized 

 by Toyama 10 in hybrids between the common Japanese 

 white silk-worm and the Siamese striped silk-worm in 

 both of which a "pale," unmarked type is latent by 

 hypostasis. The F l is uniformly striped like the 

 Siamese, and the F 2 consists of striped, "white," and 

 ' ' pale ' ' in the ratio 12 : 3 : 1. Toyama 's statement that the 

 "pale" character was in the "dormant" state indicates 

 a misconception of the nature of latency due to hypostasis. 



LATENCY DUE TO FLUCTUATION 



Another very potent cause of latency is to be found in 

 fluctuation. It is well known that many of the less 

 marked qualities of plants do not appear under unfavor- 

 able conditions of growth. By growing the offspring of 

 these poorly developed individuals under favorable condi- 

 tions they may be shown to possess all the characters of 

 other members of the race to which they belong. In- 

 visibility produced by this cause may be called latency 

 due to fluctuation. Patency is brought about by good 

 feeding, room for full individual expression, etc. As a 

 specific example, I may mention my experience with 

 several biotypes of Bursa bursa-pastoris (L.) Britton. 

 These differ from one another by certain characteristic 

 lobings of the leaves, and tjiese characters have proved, 

 on crossing, to be typical Mendelian unit-characters. 

 However, by growing the plants belonging to any of the 

 several biotypes under sufficiently unfavorable conditions 

 they may be made to produce seeds while bearing only 

 the unlobed juvenile type of leaf. The Mendelian rosette 

 characters are then wholly invisible or latent. If the 



10 Toyama, K. Studies on the Hybridology of Insects. I. On some silk- 

 worm crosses with special reference to Mendel's Law of Heredity. Bull. 

 Coll. Agr. Tokyo Imp. Univ., 7, pp. 259-393, pis. VI-XI, July, 1906. See 

 pp. 348-353 and pi. X, III, a, fe, and c. 



