314 A TEXTBOOK OF BOTANY [Ch. VI, 8 



different individuals. Expressed otherwise, and somewhat 

 fancifully, individuals are simply temporary kaleidoscopic 

 combinations of the various determiners belonging to the 

 species, the act of reproduction, especially the reduction di- 

 vision and subsequent fusion, providing the new turn of the 

 kaleidoscope. 



Thus much for heredity, which means the resemblances of 

 individuals to their ancestors. What now of variation, which 

 means the differences? The chromosome mechanism ex- 

 plains heredity well, but not variation. Indeed the mechan- 

 ism seems to leave no room for variation, since by its oper- 

 ation all individuals are simply combinations of determiners 

 which preexist. Yet variation is as real a fact as heredity, 

 for organisms do change with time, as proven by comparison 

 of living plants and animals with their fossil ancestors. 



The conception of variation, however, needs definition, 

 for some apparent variation is not at all important in evolu- 

 tion. Thus, individuals are often strongly altered in their 

 development by their conditions of life, — insufficient or 

 peculiar food, etc., and also often become altered by self- 

 adjustment to the conditions of their immediate surround- 

 ings, as we have noted already under various phases of 

 irritability. But such changes (called fluctuations) are 

 known not to be hereditary, that is, they affect the cyto- 

 plasm but not the determiners in the chromosomes. The 

 variations (called genetic variations, or mutations), 

 which produce hereditary alterations in organisms, must 

 affect the determiners, either by interpolating new ones, 

 or by altering the character or relations of those already 

 present. Yet while such mutational variation undoubtedly 

 exists, we have no knowledge as to how it arises or in what 

 way it affects the determiners. Indeed the origin of varia- 

 tion is the great crucial problem of present-day Biology, 

 though it will be settled, and before long, by the experi- 

 ments now in progress. It is the watching understand- 

 ingly for the answer to such deep questions which gives to 



