THE EVOLUTION OF SEX 19 



It would take us too far from our immediate subject 

 to attempt to discuss this matter, but it has been nec- 

 essary to refer to it in passing, for it lies at the founda- 

 tion of all questions of heredity and even involves, as 

 we shall see later, the question of heredity of sex. 



This brings us back once more to the provisional 

 conclusion we reached in connection with the experi- 

 ments on paramoecium. When the egg is fertihzed 

 by the sperm, Fig. 5, the result is essentially the same 

 as that which takes place when two paramcecia fer- 

 tiUze each other. The sperm brings into the egg a 

 nucleus that combines with the egg-nucleus. The new 

 individual is formed by recombining the hereditary 

 traits of its two parents. 



It is evident that fertilization accomplishes the same 

 result as conjugation. If our conclusion for paramoe- 

 cium holds we can understand how animals and plants 

 with eggs and sperm may better readjust themselves 

 now to this, now to that environment, within certain 

 limits. But we cannot conclude, as I have said, that this 

 process can make any permanent contribution to evolu- 

 tion. It is true that Weismann has advanced the hy- 

 pothesis that such recombinations furnish the materials 

 for evolution, but as I have said there is no evidence 

 that supports or even makes plausible his contention. 



I bring up again this point to emphasize that while the 

 conclusion we arrived at — a provisional conclusion at 

 best — may help us to understand how sexual repro- 

 duction might be beneficial to a species in maintaining 

 itself, it cannot be utilized to explain the progressive 

 advances that we must beUeve to have taken place 

 during evolution. 



