120 GENETIC VARIATIONS 



materials on which the characteristics depend. If the acquired 

 characters are indeed inherited, this means that the modified 

 organic materials increase and multiply in their modified 

 condition. 



The materials which determine characteristics and their 

 inheritance are, as we know, in the nucleus; they are what we 

 have called the genie materials or genes. Less is known of the 

 genie materials in the Protozoa than in the higher organisms. 

 But it is known that in the Protozoa there are genie materials 

 in the nucleus, and that these control in large measure the 

 characteristics of the individuals. By changing the genie 

 materials of the nucleus, and leaving the rest of the bodily 

 material the same, the inherited characteristics of the entire 

 individual are changed. This has been shown to be true in 

 unicellular organisms, through the processes that occur at 

 conjugation. In conjugation, two individuals unite and ex- 

 change halves of their nuclei (see Figure i8). Just before 

 conjugation, the single nucleus in each individual divides 

 into two halves. Then one-half of the nucleus of individual A 

 passes into B', and one-half of the nucleus of B passes into A. 

 The two halves in each then unite to form a single nucleus. 

 The two individuals then separate. 



Thus after conjugation the individual A retains its original 

 cytoplasmic body, and half its original nucleus, but contains 

 in addition a half nucleus from 5; and a corresponding 

 change has taken place in B. Now both A and B begin to 

 divide and produce descendants. 



Now, the characteristics shown by these descendants prove 

 that the transfer of half a nucleus from B carries with it the 

 power to produce in A the characteristics of B. For after 

 conjugation, the descendants of A are found to be, in the long 

 run, as much like B as they are like A. The half nucleus from 

 B produces a very great change in the characteristics of the 



