466 



THE MAINTENANCE OF SPECIES 



characterizes the succession of generations, there is no way by which 

 the laws of inheritance may be detected. Distinctive alternative 

 characters must be introduced from unlike parents and combined in 

 various ways in order to make the manner of inheritance in the 

 progeny recognizable. Transitory environmental variations, since 

 they play no part in inheritance, only cloud the picture. It is germ- 

 plasmal variations alone that can be of service in inheritance, and 

 such variations are provided in double measure by the device of 

 sexual reproduction. Thus sex is not only the major means by which 

 inheritance is effected but it also furnishes the key that unlocks the 

 mystery of how evolution is brought about. 



The way in which sexual recombination can change the flow of 

 germplasm from one generation to another is suggested in the figure, 



Two different biparental streams of germplasm, A and B, may form four new 

 different biparental streams of germplasm, a, b, c, and d, in the next generation. 



which reduces the matter to terms so simple that it is consequently 

 entirely inadequate to represent the actual complexity and possible 

 rearrangement accompanying sexual reproduction. 



Although Mother Nature's children, that is, plants, animals, and 

 even mankind, have successfully utilized the mechanism of sex for 

 an incomprehensible span of time, it is only in recent years that man 

 has come to understand, with anything like scientific accuracy, the 

 way in which it works. 



In the eighteenth century, the "ovists" held that the egg was the 

 all-important factor, and that the sperm simply served to start the 



