Zoological Frohlems for Natural History Soeiety. 9 



we should have seen that so long as it inhabited the same 

 area, it remained one, just as the English nation has 

 remained one, although its general character has slowly 

 changed, just as species have become modified in the 

 lapse of time. What has led to the breaking up of a 

 species into several has been the migration of parts into 

 new areas and the isolation of these groups of individuals 

 from the main stock ; in a word, the sending out of 

 colonies. The new conditions under which the colonists 

 live make them, to a certain extent, different from the 

 parent stock. Bateson's experiments show that there can 

 be differentiation even where the territories of the motlier 

 country and the colony adjoin, and that, when this 

 differentiation has reached a certain pitch, it is preserved 

 from the fact that the characters of the two varieties will 

 not mix in breeding. 



The science of conchology is a branch of natural history 

 eminently characteristic of the older period. It was an 

 easy and agreeable task to collect shells ; their colors 

 were pretty, and the external characters were well 

 marked. The passion for making new species out of 

 all sorts of chance variations raged unchecked, and the 

 number of species has reached portentous dimensions. 

 Now, the question is, how far have these species deter- 

 minations any real validity ? and this question is only to 

 be settled by systematic study on the spot. Owing to the 

 dif&culty of rearing fresh water molluscs, it is hardly 

 possible to apply the test of interbreeding, but it might 

 be at least ascertained how far the so-called species are 

 really distinct and whether they do not grade into each 

 other. Another most important and interesting point is 

 to ascertain how far differences in the structure of the 

 animals are correlated with the external differences in the 

 shell. In the course of my expeditions to collect 

 material for the classes in McC4ill, I have made some 

 interesting observations on our two common species 



