chief Generic Types of the Pahvozoic Corals. 307 



No one of these characters, however, can be safely relied upon 

 as, of itself sufficient for generic distinction ; and it seems 

 certain that genera should be founded upon a due consideration 

 and estimation of all the morphological characters of the 

 corallum. 



As investigated by the method which we have employed, 

 the corals of the Palaeozoic period very strongly support the 

 view that they owe their structural peculiarities to some form 

 or another of evolution. Whenever a sufficiently large number 

 of specimens of any given group can be obtained, and sub- 

 mitted to examination by means of sections, a complete 

 passage is usually found to exist into other allied groups. So 

 complete is this transition, and so gradual are the steps by 

 which it is effected, that it becomes impossible for the most 

 painstaking observer to draw any hard-and-fast line of demar- 

 cation between such inosculating groups. On the contrary, 

 he is constantly confronted with examples that might be in- 

 differently referred to one or the other of two groups, and which 

 he therefore finds it impossible to place definitely otherwise 

 than as central links in a connected series. 



Whilst the important result just enunciated is in many 

 instances capable of complete demonstration, it nevertheless 

 remains certain that in all such anastomosing groups it is 

 possible to pick out certain examples which may be regarded 

 as representatives or type forms of the groups. Such forms 

 constitute centres from which their respective groups diverge 

 in different directions ; and they are not only constant in their 

 characters, but differ from the intermediate forms in being 

 readily and certainly recognizable, and capable of easy refer- 

 ence to a definite position. 



It seems almost unnecessary to mention that these results 

 are not only precisely what would have been expected upon 

 any theory of Evolution, but that they are in complete accor- 

 dance with the results arrived at by observers in other depart- 

 ments. We may instance more especially the Foraminifera, 

 the Ostracoda, and the Graptolitida? in support of this state- 

 ment. Upon any theory of Evolution, allied groups must be 

 linked together by a more or less crowded series of intermediate 

 forms. The advocates of all hypotheses of this nature are 

 therefore compelled to hold that genera and species are merely 

 arbitrary divisions, and that they cease to have any existence 

 in reality the moment we obtain a complete series of transi- 

 tional forms. 



It still remains to consider the course which should be 

 adopted in practice, when we come to deal with these con- 

 nected and inosculating groups for the purpose of systematic 



22* 



