CLASSIFICATION A TREE-LIKE STRUCTURE. 6$ 



in the case of closely related species, connecting 

 links may be found, for they are of so recent origin 

 that the common form from which they have de- 

 scended may not in all cases have become extinct. 

 But when a wide difference exists, such as that be- 

 tween the different sub-kingdoms, the common point 

 of union is so far back in the past that the existence 

 of connecting links to-day would be a marvellous 

 thing. Such forms must have lived at the very be- 

 ginning of or even before the Silurian age, and if 

 they were found to-day it would indicate a survival 

 through periods longer than any form has been 

 known to exist. They would thus be obstacles 

 rather than arguments for the descent theory. And 

 to take another somewhat different case : the de- 

 mand for the " missing link " between man and the 

 ape, at one time so strenuously urged, has been 

 nearly abandoned, for it is recognized that the very 

 fact that man has arisen, necessitates the disappear- 

 ance of his primitive ancestor. 



But evidently these links must have existed at 

 some time, if not to-day, yet in the past. We 

 ought to expect, therefore, that the various types, 

 although not connected by living forms, are con- 

 nected by fossil forms. This question is treated 

 more at length in another chapter, and it is here 

 only necessary to say that even by fossils the sub- 

 kingdoms are not connected. No fossils have been 

 discovered which unite the sub-kingdoms with each 

 other, the earliest animals known belonging to one 

 or another of the types ; a fact which forms one of 

 the chief difficulties which evolution has to meet. 



