48 ORGANIC EVOLUTION 



time of reproduction is usually the thing that determines 

 which individuals shall mate with one another. Of prefer- 

 ence or choice, of course, there is nothing. But among some 

 of the higher animals there is evidence that individual choice 

 is exercised in the selection of mates. Breeders of domestic 

 animals find that the females sometimes prefer certain mates 

 rather than others. To quote Lloyd Morgan : " Professor 

 Low, one of the greatest authorities on our domestic animals, 

 says, ' The female of the dog, when not under restraint, makes 

 selection of her mate,' and again, ' The merino sheep and the 

 heath sheep of Scotland, if two flocks are mixed together, 

 each will breed with its own variety.' Mr. Darwin has 

 collected many facts illustrating this point. One of the chief 

 pigeon fanciers in England informed him that, if free to 

 choose, each breed would prefer mating with its own kind. 

 Darwin was informed by the Rev. W. D. Fox that his 

 flocks of white and Chinese geese kept distinct." Many 

 other instances of preferential mating could be mentioned 

 among domestic animals. To some further illustrations we 

 will refer in connection with the description of segregation. 

 Among wild animals, also, choice of mates can be observed. 

 Phenomena which are often explained by sexual selection 

 are found in some kinds of insects, among spiders, and 

 among fishes, Amphibia, reptiles, birds, and mammals. 

 Among humankind sexual selection is, of course, an impor- 

 tant factor in evolution. 



The birds give us some of the best examples of sexual 



sexual selection the fighting between the males for the possession of the female, 

 though this may have no connection with any exercise of choice on the part of the 

 female. I would include this rather under natural selection, restricting the term 

 sexual selection to the voluntary choice of mates by either the female or the male. 



