40 Darwin, and after Darwin. 



of the different modes in which efficient isolation can 

 be secured. For, in whatever degree the process of 

 organic evolution has been dependent upon heredity 

 with variability, in that degree must it also have been 

 dependent upon the means of securing homogamy, 

 whereby alone the force of heredity can be made to 

 expend itself in the innumerable directions of pro- 

 gressive change, instead of continually neutralizing the 

 force of variability by promiscuous intercrossing. 



