CH. II] MOLLUSCA AND ZOOLOGICAL REGIONS. 83 



that prevail with regard to their proper classification. 

 So intimately connected are the various types that it is 

 difficult to define the groups ; and all classification has to 

 go upon that most unsatisfactory plan, the averaging of a 

 number of structural characters. A thinning out of the 

 numberless existing genera might reveal the main lines 

 of ascent just as the branches of a tree stand out more 

 plainly when they are denuded of the leaves. I have 

 attempted to show elsewhere (p. 81, 82) that it is difficult to 

 get any real justification for the generally adopted regions 

 by the study of the range of different families. All that 

 can be said in most cases is that the various regions have 

 each a certain number of peculiar genera and families; 

 but those facts lose value from the impossibility of 

 making definite statements about the mutual affinities 

 of the genera in question. The Gallinaceous birds which 

 are less endowed with power of wing than many other 

 groups offer the best evidence of a connection between 

 structure and distribution. So too the Struthious birds. 



Land Mollusca would appear on many grounds to be 

 exceedingly valuable as furnishing evidence in favour 

 of ancient land connections. 



In an interesting paper Prof. Kobelt, a great authority 

 upon the group, urges their claims to be put in the fore- 

 front of animals useful for this purpose. 



There is however more than one serious objection 

 to their use. As Mr Blanford points out, we are not 

 at present in a very forward state of knowledge as to 

 what conchologists term " the animal " ; the anatomy 

 of the group is for a large part ignored by those even who 



62 



