36 INFECTIONS OF THE DIGESTIVE TRACT 



of microorganisms, it is also true that the biological 

 characters of these microorganisms are not the same at 

 all times of life. If we compare the bacterial flora of 

 apparently healthy persons of about the same age ; we 

 may find considerable differences in those parts of 

 the digestive tract that are easily accessible to study, 

 and yet, upon the whole, the resemblances in the bio- 

 logical characters of the bacteria so compared will 

 ordinarily be more striking than the differences, pro- 

 vided the comparison be made between the flora of in- 

 dividuals living in a temperate climate and on the kind 

 of food that is usual in countries commonly regarded as 

 civilized. But if it were practicable to compare the 

 bacterial flora of the same individual at different periods 

 of life, it would be found that there are wide and 

 perhaps characteristic variations variations dependent 

 in part on differences in food, but in part on other 

 influences. Such comparisons have probably seldom 

 been made in the same individual over a long period 

 of time, but comparisons between different persons of 

 unequal ages, but in general similarly conditioned, have 

 been repeatedly made and support the statement that 

 the normal bacterial flora characteristic of different 

 ages, present different biological characters and are 

 responsible for different types of decomposition in the 

 digestive tract. 



