194 VARIATION AND HEREDITY 



cholera bacteria and the pneumonia coccus 1 . Villinger obtained a permanently 

 immotile form of the motile Bacterium coli*, while Wasserzug developed a perma- 

 nently motile variety of the non-motile Bacillus prodigiosus 3 . 



It is uncertain whether an increased power of resistance produced by accommo- 

 dation to poisons, high temperatures, concentrated solutions, &c., can become 

 hereditarily fixed, for in experiments performed hitherto only a more or less 

 transitory after-effect has -been observed. Ray, for instance, observed that in 

 a few successive generations of Sterigomatocystis alba certain peculiarities induced 

 by cultivation on a special medium still remained perceptible 4 . Further confirma- 

 tion is, however, needed for Elfving's statement that a permanent variety of Eurotium 

 herbariorum reproducing by yeast-like cells is readily produced 5 . 



The embryonic cells of somatophytes are not only influenced by the 

 external conditions, but also by the adult cells and tissues. It is generally 

 agreed that the latter influences may be such as to favour a saltatory 

 variation, but doubt exists as to whether a gradual purposeful change can 

 be directly produced in this way, and whether properties acquired by the 

 somatic parts can become hereditary in the germinal cells 6 . Although 

 botanical evidence points to the possibility of such transference, it must be 

 admitted that the proof is not absolutely certain. 



In the growing points of plants, the progress of development is 

 directed by influences radiating from the preformed parts, and hence 

 gradual hereditary fixation is as readily possible here, as it is in asomato- 

 phytes by the progressive summation of effects due to unusual but constant 

 external conditions. Somatic properties might also be transferred to the 

 embryonic cells symbiogenetically, that is, by the entry of somatic living 

 particles or determinants. All these considerations apply to the sexual cells 

 which are derived from meristematic apices, although it is possible that 

 the induced conditions might be obliterated during maturation, or that in 

 other cases the germinal cells alone might undergo variation. 



The character of the internal changes producing variation are quite 

 unknown, but it may be remarked that the limit between gradual and 

 saltatory variations is an artificial one. For instance, a transitory high 

 temperature may immediately suppress the power of pigment-formation in 

 a bacterium and so produce a sudden variation, which at lower tem- 



1 Cf. Fliigge, Mikroorganismen, 3. Aufl., 1896, p. 479. 



2 Fliigge, 1. c., p. 489. 



3 Wasserzug, Ann. d. 1'Inst. Pasteur, 1888, II, p. 82. 



* Ray, Rev. gen. d. Bot., 1897, T. ix, p. 283. Errera (Bull. d. 1'Acad. royale d. Belgique, 

 1899, p. 99) observed an after-effect of this kind when Aspergillus niger had been grown on 

 concentrated solutions. 



8 Elfving, Einwirkung d. Lichtes auf Pilze, 1890, p. 134. 



Cf. Hertwig, Die Zelle u. d. Gewebe, 1898, II, p. 237 ; Delage, L'he"redite, 1895, p. 796; 

 Waldeyer, Verh. d. Ges. deutsch. Naturf. und Aerzte, 1897, p. 81, and the literature there 

 quoted. 



