220 Darwin, and after Darwin. 
types? Even if such a case stood alone, it would be 
strongly suggestive of error on the part of the special 
creation theory. But let us take another case, this 
time from fresh-water faunas. 
Although the geographical distribution of fresh- 
water fish and fresh-water shells is often surprisingly 
extensive and apparently capricious, this may be 
explained by the means of dispersal being here so 
varied—not only aquatic birds, floods, and whirl- 
winds, but also geographical changes of water-shed 
having all assisted in the process. Moreover, in 
some cases it is possible that the habits of more 
widely distributed fresh-water fish may have origin- 
ally been wholly or partly marine—which, of course, 
would explain the existing discontinuity of their ex- 
isting fresh-water distribution. But, be this as it 
may (and it is not a question that affects the issue 
between special creation and gradual evolution, since 
it is only a question as to how a given species has 
been dispersed from its original home, whether or 
not in that home it was specially created), the 
point I desire to bring forward is, that where we 
find a barrier to the emigration ot fresh-water 
forms which is more formidable than a thousand 
miles of ocean—a_ barrier over which neither 
water-fowl nor whirlwinds are likely to pass, and 
which is above the reach of any geological changes 
of water-shed,—where we find such a barrier, we 
always find a markcd difference in the fresh-water 
faunas on either side of it. The kind of barrier 
to which I allude is a high mountain-chain. It 
may be only a few miles wide; yet it exercises a 
greater influence on the diversification of specific 
