CONSTANCY AND MUTABILITY OF FUNGI. 673 



species, till more accurate knowledge of the relations 

 between them has heen obtained. 



Further, some plants show differences in their be- 

 haviour at times, which may be designated as modifica- 

 tions, and usually as abnormalities of lesser or greater 

 degree. These changes are set up by some unusual exter- 

 nal influence; injuries and mechanical insults, abnormal 

 nutriment, unfavourable situation, and many other causes 

 act in this way, either singly or together. The conse- TvTon- 

 quences are degenerative and involution changes of the abnormalities, 

 most various kinds visible to the eye, and abnormalities 

 in their physiological behaviour. The variations in the 

 percentage of ash in the plants, the ofttimes enormous 

 accumulation of silicic acid, the pallor of plants when 

 the nutriment is free from iron, the accumulation of 

 amides in starving flowering plants, the paler colours of 

 many flowers, and so on, are alterations of this kind set 

 up by external conditions. But it is characteristic with 

 regard to these alterations that they are inconstant and 

 non-hereditary attributes of the plants ; they only last so 

 long as the external conditions which produce them are 

 in action, and they disappear after a few generations if 

 completely normal conditions have been present. These 

 modifications are thus so variable that they cannot serve 

 for the formation of new species, for which, on the con- 

 trary, constant characters, which are transmissible through 

 a long series of generations, are necessary. 



In the third place, however, it must be admitted that 

 the characters of the species which appear to us as con- 

 stant may undergo a change after the lapse of a consider- 

 able period of time. Among many similar plants exposed 

 to the same external conditions some individuals at 

 times show slight differences'; the subsequent genera- 

 tions of these plants retain these differences ; after a 

 considerable time fresh differences may be observed 

 among some of the descendants, and in this way we 

 have a gradual formation of new varieties and species. 

 This ultimate result is attained either when the varia- Mode of origin 

 tions are of such a kind that the plants affected by them 

 grow more strongly and more quickly under the ordinary 



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