288 D. F. JONES. 



In maintaining two opposing tendencies Nature is not neces- 

 sarily working at cross-purposes. Biparental inheritance with 

 the inclination towards exogamy serves to bring about plasticity. 

 The preferential mating of similar kind operates to make this 

 mixing discriminative. Probably it is not yet time to reconcile 

 completely these two contradictory forces. It may at least be 

 held that assortative mating which favors the pairing of like 

 with like has some importance in evolution since it is an agency 

 in orthogenetic changes. Perceptive reproduction of this kind 

 tends to hold organisms in certain paths once a break from the 

 common type has been made. Having been demonstrated in 

 three of the farthest separated branches of the organic world- 

 angiosperms, protozoa, and man homogamy may take a some- 

 what more authoritative part in evolutionary theory. 



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