Effect of Mating 107 



not be settled by any conceivable experiment is not part of 

 science. 



So the question in which men have been interested in re- 

 lation to mating and fertilization is: What difference does 

 it make whether this occurs or not? This is strictly a ques- 

 tion of observation and experiment, on the same footing as 

 the question : What difference does it make whether animals 

 take food or not? 



When we ask this question regarding the union of two 

 individuals or parts of individuals which we call mating and 

 fertilization, we find that there is hardly another phenomenon 

 in biology that so alters the whole face of things. Biology 

 would be a relatively simple subject if there were no periodic 

 unions of diverse individuals, with the accompanying proc- 

 esses. This union has results of so many different kinds, 

 some immediate and obvious, others remote and hidden, that 

 we find little agreement in the accounts of its fundamental 

 features given by different investigators. 



A picture of what happens in the higher organisms that 

 we are familiar with will bring the question sharply before 

 us. Mating here involves two diverse individuals that we 

 call male and female, and two diverse germ cells, which we 

 likewise call male and female. But this is not the end; 

 the final mating is between certain parts of the cell, after 

 the two germ cells have joined to form one. The cell 

 now contains a set of pairs of small visible packets of 

 chemicals, the chromosomes (Figure 29). These mate in 

 pairs (Figure 29, D, E, F) and again separate. This is the 

 final and elementary action of mating; the union of these 

 chromosomes contains the secret of sex and of mating. 

 The union of the two diverse germ cells forms the starting 

 point for the development of the new individual. 



Several questions of general interest come into view in 



