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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 palaeontological evidence was discussed which 
seems to show that the organisms belonging to this primary 
fresh-water series are, when speaking palaeontologically, 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- 
