The Theory of Evolution 39 
THE GEOLOGICAL EVIDENCE 
On the theory of descent, as well as on the theory of 
transmutation, the ancestors of all present forms are sup- 
posed to have lived at some time in the past on the surface 
of the earth. If, therefore, their remains should have been 
preserved, we should expect on thé descent theory to find 
some, at least, of these remains to be like present forms, 
while on the transmutation theory we should expect to find 
most, if not all, of the ancestral forms to be different from 
the present ones. 
The evidence shows that fossil forms are practically all 
different from living forms, and the older they are the 
greater the difference from present forms. In general, 
therefore, it may be said that the evidence is in favor of the 
transmutation theory. It can scarcely be claimed that the 
evidence is absolutely conclusive, however probable it may 
appear, for the problem is complicated in a number of ways. 
In the first place, there is convincing evidence that some 
forms have been entirely exterminated. Other groups have 
very few living representatives, as is the case in the group 
containing nautilus, and in that of the crinoids. It is there- 
fore always possible that a given fossil form may represent 
an extinct line, and may be only indirectly connected with 
forms alive at the present time. Again the historical record 
is so broken and incomplete in all but a few cases that its 
interpretation is largely a question of probability. We can 
easily conceive that it would be only in very exceptional 
cases that successive generations of the same form would be 
buried one above the other, so that we should find the 
series unbroken. This is evident not only because the condi- 
tions that were at one time favorable for the preservation 
of organic remains might not be favorable at another time, 
but also because if the conditions remained the same the 
organisms themselves might also remain unchanged. A new 
