122 PLANT PHYSIOLOGY 



however, in an experiment the external factors very often vary from the average 

 much more than they do in nature, so also do the variations so experimentally 

 induced ; we have not in every case to deal with simple quantitative varia- 

 tions from the mean qualitative variations may also make their appearance. 

 The effect of intense light on the one hand and of complete darkness on the 

 other, or of cultivation on land or in water, in dry or damp air, must be borne 

 in mind in this connexion. It is quite impossible to express in a determinate 

 manner such variations as exhibit themselves under conditions apparently 

 constant and capable of statistical expression. Hence the statistical method 

 cannot give us any sort of picture of the possibilities of variation in plants 

 (KLEBS, 1903) ; it is only when one has cultivated a species under every con- 

 ceivable variety of external condition that one can obtain a notion of the 

 range of variation or the limits of the power of varying from the statistical 

 point of view it is only the range of variation under ordinary conditions that 

 is disclosed. 



The question now before us is whether the individual variation can play 

 a part in the formation of species ; and this question is associated with another 

 as to the inheritance of such variations. Certain agricultural experiences do 

 appear to support such an inheritance. By careful selection of individuals 

 which exhibit in a high degree a certain feature and by breeding from such 

 forms one can accentuate this character by continued selection. Thus, in the 

 sugar-beet industry, the selection and employment for propagation of the 

 seeds of those plants which are richest in sugar leads to a marked rise in 

 the general average of the sugar percentage in the beet. Fifty years ago the 

 percentage was 7-8 per cent., now it has been raised to about 15 per cent. In 

 the same way, by rigorous selection, races may be produced which will exhibit 

 especially large flowers or fruits, better flavour, increased succulence, &c. So 

 far as we know, no new characters arise spontaneously, although those already 

 existent may be added to or reduced. The limits of such variations are usually 

 reached in a few generations (3-5) ; further selection merely serves to fix the 

 character which has been acquired. It must be specially noted, however, that 

 such characters are liable to fade away quite as rapidly as they appear ; after 

 a few generations, if selection be discontinued, the original condition is reverted 

 to. Herein lies a great distinction between artificially produced races and 

 natural species : the former are transitory, the latter are constant. 



If then every variation which is the result of some external influence is 

 really to be referred to an impression made on the protoplasm of the sexual 

 cells, in consequence of which these are able in the next generation to cause 

 the appearance of a similar variation without the external stimuli being to 

 hand, and to accentuate that variation when the stimulus is present, then it is 

 difficult to see why such an effect should not be still more capable of accentua- 

 tion in the third generation. In fact, other experiences have shown that such 

 external impressions are not inherited. KLEBS notes that Saprolegnia, after 

 growing for years in a vegetative manner, proceeds to form reproductive 

 organs just as readily as a specimen grown in the ordinary way, as long as the 

 conditions necessary for the construction of such organs are granted. The 

 same is true of yeast which, in spite of continued vegetative multiplication, 

 has not lost its power of producing spores. Finally, we may draw attention 

 here to the fact that plants which have lived in the high Alps for thousands 

 of years, and which have adapted themselves to their surroundings by taking 

 on very characteristic forms, lose all these peculiarities when they are culti- 

 vated in the plains below. Conversely, lowland plants transplanted to an 

 alpine habitat take on an alpine form, but lose the adaptations which they 

 thus acquire when once more brought back to their original home (BONNIER, 

 1895). The results of selection in cultivated plants may also be otherwise 



