‘ 
Lhe 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 
