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 THE ORIGIN OF SPECIES 



CLASSIFICATION 



From the earliest times, as shown by the Mosaic story, thinking 

 man has recognised the existence of different sorts of animals and 

 has felt the necessity of giving them names. It is these nameable 

 sorts which the zoologist now calls species ; at an elementary 

 level there is no difficulty about what we mean by the term, for 

 cats are clearly distinct from dogs, and no one expects cats to 

 produce puppies or dogs to produce kittens. Cats are even clearty 

 distinct from tigers, although there is so much similarity between 

 them that the description ' big cat ' applied to a tiger seems natural 

 and obvious. When animals are more widely studied difficulties 

 appear. There are obviously several different sorts of cat : tame 

 cats and wild cats, tabby cats and ginger cats, Manx cats and 

 tailed cats, and the number of species of cat, whether one or 

 two or many, becomes a matter of opinion. No satisfactory 

 definition of ' species ' has ever been produced, and for that reason 

 experience in the study of the animals concerned is very import- 

 ant ; hence it has been said that a species is that which is 

 considered as such by a competent authority. This is a half-truth, 

 but more important is the fact that even authority must work 

 to some rules. All the members of one species must have most of 

 their features in common ; these features must be consistently 

 transmitted from parent to offspring, or at least to their remoter 

 issue ; and, if there is sexual reproduction, two individuals of 

 the same species and of opposite sex should be capable of fertile 

 union. It is on the last point that disagreement is least likely 

 to be resolved, for a decision must often be made without the 

 possibility of putting the matter to experimental proof. No one, 

 for example, has ever tried to find out if all the possibly con- 

 specific birds living in Spain and China will interbreed, but mean- 

 while a working decision about their specific status must be made. 

 Moreover, the breeding criterion does not work in reverse, for 

 there are plenty of examples of interbreeding in captivity 

 between animals which no one regards as anything but different 

 species ; species hybrids between a number of different pheasants, 



for instance, are completely fertile. The criterion that we are 



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