SUPPLEMENT 125 



a stimulus, inasmuch as the growing point is not itself in contact with the 

 water, and can only be indirectly affected by it (p. 339). Similarly, in other 

 cases, as when leaves take on forms adapted to light or to shade, the relation- 

 ship of light to the growing point enclosed in its scales must be the same in 

 both cases, although a quite different type of leaf is differentiated in the bud 

 of the shaded branch from that of the illuminated one. Experimental investi- 

 gations on this subject are, however, much required. As NORDHAUSEN (1903) 

 has shown, the characteristic anatomical differences between light- and shade- 

 leaves of the beech are already established in the bud, and the light relations 

 concerned in the unfolding of the bud play only a limited part in the process. 

 If we arrange that the primordia of the light-leaf are allowed to develop in 

 the dark, the typical double row of palisade-cells (Fig. 115, I) is retained, 

 whilst the shade-leaf retains its own characters, although it be developed in 

 bright sunlight (Fig. 115, II). The growing point, in this case, undergoes 

 adaptation, and the effect outlasts the stimulus, so that one may readily con- 

 ceive how hereditary races may in this way come into existence. As a matter 

 of fact, this is not the case in the beech ; further research is needed to show 

 how far the after-effect is continued whether the branches which have been 

 exposed to light for ten years form leaves of the light-type, after shading, 

 longer than those which have been exposed to intense light for one year. The 

 conception we hold as to inheritance of acquired characters may be more 

 briefly expressed by saying that there are no acquired characters in the 

 sense indicated ; the characters do not appear in the soma (compare GOTTE, 

 1898), but in the growing point, and hence their inheritance is taken for 

 granted. In lower organisms, which, as a rule, show no distinction into 

 somatic and germ-plasma, this is quite obvious, and it is just on these forms 

 that the observations quoted above have been made as to the inheritance of 

 effects of external factors. According to KLEBS (1906) it would appear not 

 improbable that phenomena of a similar nature must occur among higher 

 plants also. 



We have already considered the origin of races or sub-species by muta- 

 tion. The formation of species further apart out of the fundamental type is, 

 according to DE VRIES, easily understood, since many ' petite especes ' dis- 

 appear in the struggle for existence. Of course the mutation may be so great 

 that a new genus or a new family may come into existence. Capsella Heegeri, 

 for example, would scarcely have been placed in the genus Capsella were it 

 not that its origin was known. It is also quite possible that whole genera and 

 families may be referred back in their chief features to monstrosities. HILDE- 

 BRAND (1890) has found Fuchsia to produce zygomorphic flowers arising by 

 mutation (Fig. 118), so that the allied genus Lopezia may have arisen from 

 a malformation (compare SACHS, 1893). 



HOFMEISTER long ago (1868, p. 564) ascribed to mutation a very prominent 

 part in the formation of species. He said : ' New forms do not come into 

 existence by the summation in successive generations of small differences 

 from the customary form, all tending in the same direction ; they appear 

 suddenly, and are widely different from the parent.' 



In addition to individual variations and mutations, a third type of varia- 

 tion in plants has often been assumed to occur, i.e. the so-called ' adaptations '. 

 By adaptations are understood changes which subserve special purposes in 

 the organism. Such adaptations do not differ, it may be said, from other 

 variations in their mode of origin, but only in the way we look at them. From 

 a purely physiological standpoint, and looking at causal relation only, there 

 are no such things as adaptations ; they exist only as ecological (biological) 

 phenomena, when the question of purpose is under consideration. Although 

 we regard this way of looking at the subject as quite permissible, still we may 



