172 



bility a transition form, leading us onwards into some 

 neighbouring group *. 



I will not, however, venture too closely into this ques- 

 tion in its minor bearings ; suffice it to have demon- 

 strated that, whatever be the rate, law, or direction, of 

 the advancement of the various groups towards a more 

 perfect model ; or in whatsoever position the several types 

 are to be discerned, with respect to their immediate 

 associates, genera cannot be isolated and distinct, but 

 must of necessity merge (each into two or more others) 

 on their outer limits. Hence, if such be the case, as I 

 contend that it usually is (the exceptions to the rule 

 being, as I shall hope shortly to prove, the result of 

 accident, and by no means a part of the original design), 

 it may perhaps be a problem, how far we are justified in 

 rejecting many large and natural assemblages, through 

 the fact that they blend, both at their commencement 

 and termination, imperceptibly, with others, their pre- 

 cise boundaries being dimly defined. 



That the recognition of genera is necessary, even as 

 a matter of mere convenience, is self-evident; for in 

 many extensive departments they combine with each other 

 so completely at their extremities (although sufficiently 

 well-marked in the mass), that, unless we are prepared 



* I may add, that this suggestion, as to the evenly balanced state 

 of generic types, is in accordance with the views of Mr. Waterhouse, 

 whose extensive knowledge in the higher departments of zoolo- 

 gical science gives a value to his opinion, especially on questions 

 such as these, which I am glad to have an opportunity of acknow- 

 ledging. 



