II. 

 The Role of Sex. 



Part II. 



The Utility of Conjugation. 



AS already mentioned, there are two methods of reproduction, 

 asexual and sexual. In the former case, a portion of an indivi- 

 dual, generally a reproductive cell, grows and develops into an adult. 

 Thus, in certain times of the year the aphides produce fresh brood 

 without sexual assistance, and tubers and cuttings can be used in the 

 propagation of potatoes and roses. In the case of sexual reproduction, 

 matter, generally in the form of reproductive cells, is mingled by two 

 individuals, and from the resultant mass development proceeds. 

 Now, it appears that there are some material advantages to be gained 

 by the second method, for we find that long-continued asexual repro- 

 duction leads to deterioration and final extinction. Thus, trees and 

 plants produced for many years by tubers or cuttings, show marked 

 deterioration, or become subject to disease, an indication of some 

 hereditary weakness. The ciliated infusoria observed by Maupas 

 pass through periods where sexual conjugation does not occur, but 

 finally, if life is to continue, they must conjugate with individuals of 

 other stocks. Conjugation, and especially conjugation between 

 different stocks of the same species, is, therefore, in some way 

 advantageous, even necessary, and we have now to ask in what way 

 it is advantageous. 



Many naturalists affirm that the process of conjugation is a 

 rejuvenescence, and are content with this. But this is no explana- 

 tion, and cannot be viewed even as an attempt at one. It is a fact 

 that plants or animals deteriorated by long-continued asexual repro- 

 duction may make a new and vigorous start if permitted to conjugate 

 with other individuals ; but to say that this is rejuvenescence merely 

 expresses, by a metaphor of very superficial application, the fact 

 which we are called to explain. 



According to Weismann, sexual reproduction is of utility, inas- 

 much as he supposes that it " gives Natural Selection a choice of 

 innumerable combinations of the most diversified variations to act 

 upon."' "I am convinced," he says, "that the conjugation of 



1 The Germ Plasma, a Theory of Heredity. Translation by W. N. Parker ; 

 London, 1893 ; pp. 413, 431, and 4G3. 



