31 
THE SILURIAN BEACH. 
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identical forms in the neighborhood of the snow- 
fields in lofty mountains, that to trace the differ- 
ence between the ptarmigans, rabbits, and other 
gnawing animals of the Alps, for instance, and 
those of the Arctics, is among the most difficult 
problems of modern science. 
And so is it also with the animated world of 
past ages ; in similar deposits of sand, mud, or 
lime, in adjoining regions of the same geological 
age, identical remains of animals and plants may 
be found ; while at greater distances, but under 
similar circumstances, representative species may 
occur. In very remote regions, however, whether 
the circumstances be similar or dissimilar, the 
general aspect of the organic world differs greatly, 
remoteness in space being thus in some measure 
an indication of the degree of affinity between dif- 
ferent faunae. In deposits of different geological 
periods immediately following each other, we 
sometimes find remains of animals and plants so 
closely allied to those of earlier or later periods 
that at first sight the specific differences are 
hardly discernible. The difficulty of solving 
these questions, and of appreciating correctly the 
differences and similarities between such closely 
allied organisms, explains the antagonistic views 
of many naturalists respecting the range of exist 
ence of animals, during longer or shorter geologi- 
cal periods ; and the superficial way in which dis 
