FACTS CONCERNING VARIATION 



191 



modification of the racial peculiarities must take place during the formation 

 of the sexual cells, whereas if the meristem cells merely have impressed upon 

 them the special peculiarities of the adult plant, then the sexual cells and 

 the resultant embryo must be withdrawn from these inducing influences 

 during their growth and development. 



It is possible that further differences may exist as regards the propaga- 

 tion of individual peculiarities by different modes of reproduction, and there 

 may be some varieties whose peculiarities are not propagated by all forms 

 of vegetative reproduction. The restriction of the term ' heredity ' to sexual 

 propagation is neither possible nor advisable, for by this term we merely 

 indicate that property by which the characters of a plant are repeated in 

 successive generations, and leave entirely undetermined the ways and means 

 by which such repetition is effected. Furthermore, in the case of organisms 

 which reproduce asexually only, we could not speak of hereditary trans- 

 ference at all. In the broad sense, however, an instance of heredity is 

 presented us, when a cultural variety is propagated vegetatively, even though 

 it can also reproduce by sexual means. 



The problems of heredity are closely connected with the definitions of 

 species, variety, race, &C. 1 , and it has long been recognized that a species has 

 no definite boundaries, but is a conventional expression for a cycle of forms 

 grouped around a type-centre. The explanation of the origin of a plant 

 will not destroy its right to be considered as a distinct species, although 

 it does not follow that a permanent hybrid should be given specific rank, or 

 that the species of lichens have the same value as those of single plants 

 reproducing themselves from single or fused germ-cells. 



SECTION 56. Pacts concerning Variation. 



Disregarding the external or internal mode of origin, we can distinguish 

 between indeterminate, spontaneous, saltatory, or single variations, and 

 gradual, determinate, or adaptive ones. In the second case we may be 

 dealing with the gradual hereditary fixation of some specific reaction, 

 whereas a saltatory variation involves the sudden appearance of some new 

 property in one or a few individuals. Here also, however, the variation is 

 the result of the prevailing conditions, internal and external, and it is 

 readily comprehensible that unusual conditions should excite a tendency to 

 saltatory variation, and hence also to corresponding internal changes. By 

 comparison with machines we can understand that according to the agencies 

 operating, and the organisms affected, the variation may either be restricted 

 to particular individuals, or affect all to much the same degree. 



1 Cf. e.g. Niigeli, Theorie der Abstammungslehre, 1884, p. 235; Delage, L'heredite, 1895, 

 p. 627. On 'economic' species cf. Klebahn, Bot. Ztg., 1898, p. 148 ; Migula, System d. Bacterien, 

 1897, I, p. 222. 



