1 92 VARIATION AND HEREDITY 



The irregular saltatory variations known as sports have long been used 

 as the starting-points in the production of cultural varieties l , and these may 

 either tend to revert to the ancestral stock, or may become permanent forms 

 capable of vegetative multiplication or even of reproduction by seeds. 



That saltatory variations may be inherited is certain, but some 

 authors dispute the heredity of adaptive variations. Too much attention 

 is, however, usually paid to the higher somatic organisms, in which the 

 hereditary transference of acquired somatic characters is rendered more 

 difficult by the fact that they must first be communicated to the embryonic 

 cells. This is not the case in asomatophytes, and in these, by continued 

 cultivation under special conditions, particular variations may regularly 

 appear, and slowly disappear again on returning to normal conditions. 



For example, it has been found possible to produce asporogenous races 

 of certain species of bacteria and of Saccharomycetes, while the power of 

 producing poisons or pigments has been permanently eliminated from 

 certain bacteria. In this case a hereditary modification of metabolism has 

 been produced, whereas the loss of the power of spore-formation removes 

 a morphological characteristic of great value in the classification of 

 bacteria. 



The variations in question are at first labile, and only gradually acquire 

 a fixed hereditary character, for if after short exposure the organisms are 

 returned to the original conditions, the power of pigment, spore, or poison 

 production either returns at once or in the course of a few generations. In 

 other words, according to the degree of induction, the transitory after-effect 

 requires a longer or shorter time for its removal, and becomes a permanent 

 variation when the labile internal changes are rendered stable. The length 

 of time required for this depends upon the plant, and upon the nature and 

 intensity of the agency producing variation. 



Since the colourless and non-poisonous races of bacteria, as well as the 

 asporogenous yeast-forms, retain their peculiarities for years under cultiva- 

 tion, they may be regarded as fixed forms, although a reversion or new 

 variation might occur after a still longer period of cultivation under the 

 same or other unusual conditions. This actually applies to the existent 

 species which have arisen by a series of unknown variations, and in fact it is 

 doubtful whether any species has so fixed a character as to be incapable of 

 variation under all conditions and for all time. 



Experiments on asomatophytes can be readily extended over a far 

 greater number of generations than in the case of flowering plants, for 



1 See the works of Darwin, on Origin of Species, &c. Also Hofmeister, Allg. Morph., 1867, 

 p. 557 ; de Vries, Bot. Centralbl., 1899, Bd. LXXVII, p. 327 ; Biol. Centralbl., 1900, Bd. xx, p. 193; 

 Compt. rend., 1900, T. cxxxi, p. 124; Solms-Laubach, Bot. Ztg., 1900, p. 175 ; Wettstein, Ber. d. 

 Bot. Ges., 1901, Generalvers., p. 1,184; Beyerinck, On Different Forms of Hereditary Variation of 

 Microbes, 1900 (repr. from Koninklijke Akad. v. Wetenschappen te Amsterdam). 



