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CHAPTER XVI. 



In the preceding chapters of this work we considered 

 first what may be called the problem of fresh-water faunas 

 in general. That is to say, we examined the nature of 

 such faunas in the more remotely separated land-masses of 

 the globe, and by such an examination it was shown, 

 that in spite of the wide variations which often seem to 

 characterise such assemblages of living forms, there is, 

 nevertheless, clearly to be discerned, an element of 

 similarity underlying whatever apparent local diversity 

 may be presented. 



In the second place, an attempt was made to directly " get 

 at " both the meaning of this wide-spread similarity, and of 

 the local variations with which it is often accompanied. A 

 quantity of morphological evidence was cited, which seems 

 to indicate that the components of the universal fresh- 

 water series are somewhat archaic in character, and at the 

 same time palseontological evidence was discussed which 

 seems to show that the organisms belonging to this primary 

 fresh-water series are, when speaking palseontologically, of 

 the same date. In other words, there was found to be 

 evidence which indicates that most, if not all, the primary 

 fresh-water organisms became differentiated about the 

 same time. It was shewn further, that the period at 

 which the primary series emerged from the general fauna 

 of the sea, was synchronous both with a singular multi- 



