The Theory of Evolution 35 



the differences are so small that it is difficult to distinguish 

 between two forms, but even in such cases the differences, 

 although small, may be as numerous as when they are more 

 conspicuous. If, then, this is what we really find when we 

 carefully examine species of animals or of plants, what is 

 meant when we claim that our classification is based on the 

 characters common to all of the forms that have descended 

 from the same ancestor ? We shall find, if we press this 

 point that, in one sense, there is no absolute basis of this sort 

 for our classification, and that we have an unreal system. 



If this is admitted, does our boasted system of classification, 

 based as it is on the principle of descent, give us anything 

 fundamentally different from an artificial classification ? A 

 few illustrations may make clearer the discussion that follows. 

 If, for example, we take a definition of the group of verte- 

 brates we read : " The group of craniate vertebrates includes 

 those animals known as Fishes, Amphibians, Reptiles, Birds, 

 and Mammals ; or in other words, Vertebrates with a skull, a 

 highly complex brain, a heart of three or four chambers, and 

 red blood corpuscles." If we attempt to analyze this defini- 

 tion, we find it stated that the skull is a characteristic of all 

 vertebrates, but if we ask what this thing is that is called 

 skull, we find not only that it is something different in dif- 

 ferent groups, being cartilaginous in sharks, and composed 

 of bones in mammals, but that it is not even identical in 

 any two species of vertebrates. If we try to define it as a 

 case of harder material around the brain, then it is not 

 something peculiar to the vertebrates, since the brain of the 

 squid is also encased in a cartilaginous skull. What has been 

 said of the skull may be said in substance of the brain, of the 

 heart, and even of the red blood corpuscles. 



If we select another group, we find that the birds present 

 a sharply defined class with very definite characters. The 

 definition of the group runs as follows : " Birds are char- 

 acterized by the presence of feathers, their fore-limbs are 



