AMONG UNICELLULAR ORGANISMS 141 



usual number of successive generations occurred ; members 

 removed at different stages were again observed to con- 

 jugate successfully with unrelated forms, and this was 

 done to the one hundred and thirtieth generation. After 

 that, however, the family being again near its end, the 

 removal was no longer any use. About the one hundred 

 and eighteenth generation the strange sight was seen 

 of individuals of the same family attempting to unite 

 with one another. The results were, however, nil, and 

 the conjugates did not even recover from the effects 

 of their forlorn hope." * 



Here we have just such facts as the theory would lead 

 us to expect. When the cells are fresh and in full vigour, 

 conjugation is unnecessary. As they become exhausted 

 during repeated asexual generations, conjugation becomes 

 more and more necessary up to a certain point. When, 

 however, exhaustion is carried beyond that point, con- 

 jugation becomes impossible even with unrelated forms. 

 But whether the exhaustion which comes of repeated 

 asexual generations is analogous with the results of over- 

 work or under-feeding among the higher organisms is a 

 debatable question. It has been termed " senile decay." 



The effects of nutritive conditions among these lower 

 organisms parallel in a singular way what we have seen 

 to be the rule among the higher forms of life. " Thus, 

 if nourishment be continually and abundantly supplied 

 to cultures of Ciliata, pairing can be prevented (Maupas). 

 They continue to divide until the whole culture dies off 

 in consequence of senile degeneration. On the other 

 hand, cultures of Infusoria which are approaching sexual 

 maturity may be induced to pair by withholding nourish- 

 ment." 2 This is closely analogous with what occurs 

 among the higher forms of life. Surely, therefore, it 



1 Evolution of Sex, Geddes and Thomson, chap. xii. 

 1 The Cell, Dr. O. Hertwig. 



