36 Life and Death, Heredity and Evolution 



times and which does not interrupt the life of the individual. 



What now are we to think of conjugation in the light of 

 these new facts? In conjugation there occurs the same re- 

 placement of the old active nucleus by a part of the reserve, 

 which we have seen to take place also without conjugation ; 

 but with the additional fact of an exchange of pieces of the 

 new nucleus between the two conjugating individuals. If 

 this replacement means rejuvenescence when it occurs with- 

 out conjugation, there is no reason to deny it that meaning 

 in conjugation; conjugation too should result in rejuvenes- 

 cence. But what is the significance of the additional feature 

 the exchange of nuclear parts between the two conjugat- 

 ing animals? This seems not necessary to rejuvenescence, 

 since the replacement of the old by the reserve may occur 

 without it. Why then does the exchange take place? 



In many higher organisms we observe that the mating 

 and the union of parts of two different cells which we call 

 fertilization are necessary in order that reproduction shall 

 occur; the egg does not develop unless fertilized. But in 

 these Protozoa we observe that reproduction occurs for gen- 

 eration after generation without this mixture of two diverse 

 cells; and we have just seen that rejuvenescence likewise 

 occurs without such a mixture. In these lower creatures we 

 find separated therefore two processes which in many higher 

 animals are so closely bound together that we get the im- 

 pression that they are inseparable; that development neces- 

 sarily depends on fertilization. But even in many higher 

 animals development may take place without fertilization; 

 these, taken with the facts in the Protozoa, show that there 

 is no generally necessary relation between the two things. 

 It appears, as we shall see later, that the close association 

 of reproduction with a union of two cells is only a special 

 peculiarity of certain organisms: something that might be 

 called a special adaptation. 



