356 THE HISTORY OF CREATION. 
That even the origin of man must be explained by this 
general organic process of transmutation, and that it is 
simply as well as naturally explained by it, has, I believe, 
been sufficiently proved in my last chapter but one. I 
cannot, however, avoid here once more directing atten- 
tion to the inseparable connection between this so-called 
“theory of apes,” or “pithecoid theory,” and the whole 
Theory of Descent. If the latter is the greatest onductive 
law of biology, then it of necessity follows that the former 
is its most important deductive law. They stand and fall 
together. As all depends upon a right understanding of 
this proposition, which in my opinion is very important, 
and which I have therefore several times brought before 
the reader, I may be allowed to explain it here by an 
example. 
In all mammais known to us the centre of the nervous 
system is the spinal marrow and the brain, and the centre 
of the vascular system is a quadrupal heart, consisting of 
two principal chambers and two ante-chambers. From this 
we draw the general inductive conclusion that all mammals, 
without exception, those extinct, together with all those 
living species as yet unknown to us, as well as the species 
which we have examined, possess a like organization, a like 
heart, brain, and spinal marrow. Now if, as still happens 
every year, there be discovered in any part of the earth a 
new species of mammal, a new species of marsupial, or a 
new species of deer, or a new species of ape, every zoologist 
knows with certainty at once, without having examined its 
inner structure, that this species must possess a quadruple 
heart, a brain and spinal marrow, like all other mammals. 
Not a single naturalist would ever think of supposing that 
