NATURAL SELECTION 75 



ments which have been offered for and against the 

 theory. 



Darwin gives special attention to the subject of 

 variation under domestication. It is well known that 

 new varieties of plants and animals are produced 

 under domestication, and that they are separated and 

 preserved as distinct varieties by the agency of man. 



He dwells at length on the many varieties of domes- 

 tic pigeons, and attempts to show that their common 

 ancestor was the rock pigeon. These varieties, he 

 claims, are as different in structure from each other 

 as are all well-marked species. 



The varieties of pigeons all cross readily with each 

 other, producing fertile offspring, and there is a fre- 

 quent tendency in these crosses to revert to the form 

 of the original stock. This is the clearest case that 

 he presents among the domestic animals of the recent 

 origin of well-marked varieties. 



He claims that these varieties are incipient species, 

 and that species are only well marked varieties. If 

 we grant that varieties can be produced, where can we 

 set the limits of variability? 



He concludes that we cannot set a limit, and that, 

 consequently, by the law of natural selection princi- 

 pally, all organic forms have been evolved from a sim- 

 ple form. 



With regard to the fertility of hybrids he says, 

 "Now it is difficult, perhaps impossible, to bring for- 

 ward one case- of the hybrid offspring of two animals 

 clearly distinct being themselves perfectly fertile." 



It is well known that as a rule distinct species will 

 not cross, and that if they do cross the offspring are 

 not fertile. 



On the other hand, it is true that all varieties of a 

 species readily cross, producing fertile offspring. 



