THE PRINCIPLES OF BIOLOGY. 
103 
and it seems surprising that organs of such difference 
externally, and of such different uses, should be constructed 
on so similar a plan. Look at the vertebral column of a 
man. One part, the sacrum, in which great strength is 
required, consists of five vertebrae consolidated to one piece, 
but still showing the divisions. The number of these 
divisions varies in different orders and even in the same 
order. 
Can we suppose such structures to be the result of a 
design, or chance, or necessity? No. Here, as before, 
evolution is the only agency by which such a state could have 
been brought about. 
In the last chapter we saw that during development an 
organism often requires organs which later on disappear, are 
absorbed, or replaced by other organs; but often they are 
retained and form then what are called rudimentary organs. 
The examples of these are numerous; some snakes have 
rudimentary legs under the skin ; smooth-skinned amphibia 
sometimes have rudimentary scales under the skin. In some 
birds the feathers are rudimentary and are reduced to simple 
shafts or even hairlike forms. Our own fine hairs covering 
the whole of the body are the rudimentary covering homo¬ 
logous to the hair of mammals. 
There are numerous cases ol rudimentary wings in 
insects, or wings uselessly hidden under firmly closed wing- 
cases. In the productions of artificial selection we often find 
such cases; rudiments of tails in tailless breeds, rudimentary 
ears or horns in breeds which are without these organs. 
In all these cases an assumption of design would 
evidently be absurd, as likewise that of necessity, and we 
arrive again at the conclusion that they represent the signs 
of a kinship between the organisms which must be traced 
back to their common ancestors. 
We have thus three points in which all theories but that 
of evolution fail: First, the unity of type in forms of great 
difference belonging to the same group; second, the similarity 
of structure in different organs of the same organism; third, 
the presence of rudimentary and useless organs; but all can 
easily be explained by the theory of evolution and follow as 
a necessary consequence of it. 
Chapter VII. 
In considering the facts of distribution as arguments for 
or against evolution we must treat separately distribution in 
space and distribution in time; further we have to distinguish, 
when investigating distribution in space, between that over 
