SPECIES IN DOMESTIC BIRDS 25 



own kind, and so each species remains distinct ; but if in a 

 species there are many different types, such as we find in 

 domestic fowls, the members of the species, when free to do 

 so, mate as readily with types quite different from their own as 

 with individuals exactly like them, and produce offspring of in- 

 termediate types with all the essential characters of the species. 

 In domestication individuals of distinct yet similar species are 

 sometimes mated and produce offspring called hybrids, but 

 these are sterile. The mule, which is a hybrid between the ass 

 and the mare, is the most familiar animal of this kind. Hybrid, 

 or mule, cage birds are produced by crossing the canary with 

 several allied species. Among other domestic birds hybrids are 

 almost unknown. 



Origin of species. Until near the close of the last century it 

 was commonly believed that each species had been created in 

 perfect form and that species were unchangeable ; but long be- 

 fore that time some keen students of the natural sciences and 

 close observers of the changes that take place in plants and 

 animals in domestication had discovered that species were not 

 perfectly stable and were changing slowly. Geologists estab- 

 lished the fact that the earth, instead of being only a few thou- 

 sand years old, had existed for countless centuries. Among 

 fossil remains of creatures unlike any now known they had 

 found also other forms which appeared to be prototypes of ex- 

 isting species. The idea that the forms of life now on the earth 

 had come from earlier and somewhat different forms had occur- 

 red to several scientists more than a hundred years ago, but it 

 was not until about 1860 that a satisfactory explanation of pro- 

 gressive development of forms of life was given to the world. 

 This mode of creation is called evolution. 



The theory of evolution is that partly through their own 

 inherent tendency to vary and partly through the influence of 

 external things which affect them, all organisms change slowly ; 

 that things of the same kind, separated and living under different 



