60 SEX AND HEREDITY 



In the life-history of a simple Protozoon like Copro- 

 monas we have seen that there are associated together 

 two very distinct processes one of reproduction, in which 

 the individual becomes resolved into two new individuals, 

 and the other syngamy in which two individuals 

 become merged into one. The same is the case amongst 

 these lowly organized creatures in general : the zygote- 

 an individual formed by the fusion of two gametes- 

 reproduces by the simple process of fission over and over 

 again ; then syngamy takes place and the new zygote 

 goes on reproducing as before and so on indefinitely. 



The process of syngamy appears as a general rule to 

 be an essential part of the cycle. If a single individual 

 be isolated in a vessel by itself, and kept under suitable 

 conditions as to food supply and so on, it will go on 

 reproducing for it may be some hundreds of generations. 

 From time to time, however, waves of depression seem 

 to pass over the culture, during which the reproductive 

 activity becomes slackened. As time goes on these 

 waves of depression become more and more marked : the 

 vitality of the individuals becomes obviously impaired ; 

 they degenerate and undergo a kind of senile decay, and 

 eventually the whole culture dies off. 



Now it has been found that it is possible to tide a 

 culture successfully through these periods of depression 

 by making some marked change in the conditions under 

 which the culture is living, or by applying some special 

 stimulus. When this is done the individuals appear 

 to renew their vitality, and proceed to reproduce over and 

 over again as before. 



Here then we learn a very important lesson. We 

 have been accustomed to regard Death as a necessary 

 sequel to Life, but we see that amongst these lowly 



