CLASSIFICATION AND ADAPTATION 15 



The older naturalists might have said with truth: 

 we do not know what the species are, but we are 

 quite certain that whatever they are they have 

 never undergone any change in their distinguishing 

 characters. At the same time we know that whether 

 we call related forms varieties or species or genera 

 in different cases, we find, whatever organisms 

 we study, whether plants or animals, definite types 

 distinguished by special characters of form, colour, 

 and structure, and that individuals of one species 

 or type never give rise by generation to individuals 

 of any other known species or type. We do not 

 find wolves producing foxes, or bulldogs giving 

 birth to greyhounds. As a general rule the dis- 

 tinguishing characters are inherited, and it is by 

 no means easy even in domesticated animals and 

 plants to obtain an exact and complete record of 

 the descent of a new variety from the original form. 

 Among species in a state of nature it is the exception 

 to find two recognised species which can be crossed 

 or hybridised. In the case of the horse and the ass, 

 although mules are the hybrid offspring of the two, 

 the mules themselves are sterile, and there are many 

 similar cases, so that some naturalists have main- 

 tained that mutual infertility should be recognised 

 as the test of separation in species. 



Darwin founded his theory on the assumption 

 that differences of species were differences of adapta- 

 tion. His theory of natural selection is a theory 

 of the origin of adaptations, and only a theory of 

 the origin of species on the assumption that their 

 distinguishing characters are adaptations to different 

 modes and conditions of life, to different require- 

 ments. He pointed out that there is always a 



