104 Life and Death, Heredity and Evolution 



without conjugation; the modifications should therefore dis- 

 appear at such times. The fact that they do not indicates 

 that the distinction is not one depending on whether the seat 

 of the modifications is in the macronucleus or the micronu- 

 cleus ; it leaves the distinction indeed with no very intelligible 

 foundation. It appears possible that modifications which 

 are novv known to last for months might endure still longer, 

 and become as permanent as any character, if the conditions 

 producing them lasted for much longer periods. The fact 

 that the modifications sometimes disappear at conjugation 

 may be due to the fact that variations of many sorts occur 

 as a result of conjugation, as will be set forth in our account 

 of that matter. The number of cases in which these phe- 

 nomena have been studied is very small ; too small for basing 

 positive conclusions on these points. 



All together, the studies of the effects of external agents 

 on heredity in the Protozoa show that changes in the 

 hereditary characters are in this way produced only most 

 slowly and rarely. The organisms are found most resistant 

 to such changes ; any alteration produced in a given genera- 

 tion is usually compensated for in the next generation. Al- 

 most every investigator of the matter passes through a long 

 stage in which he can hardly resist the conviction that no 

 hereditary changes can be brought about in this manner. 

 But if work is continued for very long periods of time, the 

 hereditary constitution of the stock is seen to gradually 

 yield; at first only in a slight degree and with results that 

 are transitory. The differences finally become so fixed that 

 they are transmitted in the ordinary reproduction by fission. 

 After conjugation, with its extreme physiological altera- 

 tions, and production of new combinations of inherited char- 

 acters, the inherited environmental effects are frequently no 



