232 PHYSICAL BASIS OF CIVILIZATION 



ner in his or her barren mate ? In the mating of 

 individuals of closely related species, such as horses 

 and asses, is a failure to produce fertile offspring 

 owing to deficiency in the complementariness of 

 the reproductive substances or of the reproductive 

 organs? All such questions bear on the problem 

 of the origin of species, the details of which remain 

 almost as much terra incognita as in the days be- 

 fore Darwin. Speculation rationally conducted and 

 based on the known nature of things is therefore 

 legitimate, and may prove helpful in giving direction 

 to research, experiment, and discussion. 



As above indicated, there is good reason for the 

 opinion that it will some day be discovered that 

 the sporadic occurrence of barrenness in a species 

 is the forerunner of the birth of a new species; viz., 

 when variation through sexual reproduction has 

 produced deviations in reproductive cells or organs 

 or both of such a nature, that the individual affected 

 cannot reproduce with the normal type, then the 

 deviations occurring in one sex may sometimes 

 become complementary to those occurring in the other, 

 so that the individuals of one sex which are barren 

 with reference to the specific type may success- 

 fully reproduce with those of the other sex which 

 are also barren with reference to the specific type. 



The explanation above given is supplementary 

 to the Darwinian theory of the origin of species. 

 This starts out with the proposition stated in para- 

 graph 3d of Chapter I of this book, "that all higher 



