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 the 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 



