INTRODUCTION 17 



inbreeding and the traditions regarding marriage of near 

 kin are very similar. The great majority of breeders have 

 an ineradicable fear of evil consequences if their ma tings 

 are too close. Only here and there a few fearless ones 

 have used systems of extremely close mating to perpetu- 

 ate their breeds, and by such methods have built up in- 

 valuable races of horses, cattle, swine and poultry. But 

 here a dilemma appears. Inbreeding has deplorable re- 

 sults in certain cases, yet in other instances the returns 

 have been gratifying. What is to be the future practice? 

 To be' more than mere trial and error, it must be founded 

 upon a cogent analysis of the whole subject. 



Finally, interest in the effect of various systems of 

 mating as natural phenomena has been stimulated by the 

 study of organic evolution. The circumstantial data of 

 comparative morphology show that in nature problems 

 similar to those of man have arisen. If these problems 

 are investigated some light may be thrown upon his diffi- 

 culties. Sexual reproduction has been the most success- 

 ful method of providing for the propagation of animals 

 and plants. Does sexual reproduction, therefore, possess 

 an advantage over other methods! Would it otherwise 

 have persisted as it has in both kingdoms? It probably 

 was not the original method of reproduction. Asexual 

 reproduction, reproduction by simple vegetative division, 

 appears to have held the stage when animals and plants 

 were simple and unspecialized. Then, in all probability, 

 came sexual reproduction with separation of the sexes. 

 Secondarily, however, numerous species arose in both 

 kingdoms wherein the sexes are united, male and female 

 cells being produced in the same individual. Thus a sys- 

 tem of mating entailing the greatest possible amount of 



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