192 COOK 



sary the more general inferences which have been drawn from 

 Mendelism, such as the existence of character unit particles and 

 the purity of germ-cells. 



Instead of being a simple process of making two successive 

 combinations or crosses, of like nature, but with constantly dif- 

 ferent results, the reproductive processes involved in a Men- 

 delian experiment are highly complex, and very different from 

 what is commonly supposed. Before the bearings of Mendelism 

 on the general problems of descent and evolution can be appre- 

 ciated, it is necessary that the components of the phenomenon 

 be clearly distinguished. 



PARENT ORGANISxM PRODUCE PARENT GAMETES. 



It is customary to say in describing Mendelian experiments 

 that a cross was made of two individual plants or animals rep- 

 resenting certain contrasts of characters. But this simple state- 

 ment may carry us at once on to false ground, unless we follow 

 closely the details of the process. What we refer to as crossing 

 the plants or animals is merely bringing together the gametes or 

 sex-cells which these particular organisms have produced. The 

 organisms themselves are not involved in the experiment, except 

 to the extent that they have furnished the sex-cells and provided 

 nourishment for the embryos to which the sex-cells give rise. 

 It is the sex-cells or gametes with which the experiment in cross- 

 ing really begins. They are the true parents of the cross, 

 rather than the organisms selected as parents. Gametes do not 

 belong to the same generation or conjugation period as the parent 

 organisms, but represent a new generation which is already be- 

 gun before the intention to make a Mendelian or other cross can 

 become effective. 



PARENT GAMETES PRODUCE CONJUGATE ORGANISMS. 



The union of the gamete parents opens the way to the de- 

 velopment of a new organism, which it is customary to consider 

 as the first generation of the desired cross. But again we may 

 ask, what are the detailed facts of the case? 



The prevalent view has been that the production of a new 

 organic individual is the result of a conjugation, a complete 



