92 Life and Death, Heredity and Evolution 



of time that the modifying factors have acted; the longer 

 they act, the more decidedly hereditary become the changes 

 they produce. 



Many of the hereditary changes that have been produced 

 in bacteria manifest themselves only in altered physiological 

 activities. Bacteria break up many sorts of organic com- 

 pounds, obtaining by the recombination of their components 

 the energy necessary for their own vital activities. Diverse 

 species or races thus decompose different compounds. In a 

 number of cases it has been found that if bacteria of a 

 particular sort are cultivated in the presence of a com- 

 pound which they do not decompose, but which is not too 

 unlike some compound on which they can live, in the course 

 of time some of the individuals acquire the power of de- 

 composing and living upon this unaccustomed substance. 

 This power then remains hereditary, so that the descend- 

 ants have it also, even though they may be cultivated 

 under conditions in which it is not exercised. 



For example, Massini (1907) found that a certain bac- 

 terium which belongs to the group of which the typhoid 

 bacillus is a member, had not the power of decomposing 

 lactose. But if they are grown on a culture medium that 

 contains lactose, after a few days certain parts of the 

 colonies begin to grow rapidly, forming small nodules ; and 

 tests show that these are now decomposing the lactose. If 

 these are removed to other media and cultivated for many 

 generations without lactose, their descendants still retain 

 the power of decomposing this substance, as is shown by 

 replacing them on a medium with lactose. 



This fact has been confirmed by many observers, and sim- 

 ilar changes have been observed in other cases. Bacteria 

 have been caused to acquire the power of splitting up lactose, 

 dulcite, rhamnose and various other carbohydrates, though 



