14 
muddy and the other clear; but the most interesting conclusion was a 
notable difference in degree of specialization in the fishes inhabiting the 
different sections of a stream system. Those from the larger river were, 
as a rule, not only the largest, but the most primitive, or the least special- 
ized; those preferring the bottomland lakes were, on the whole, more 
highly differentiated; and those from the creeks were smallest and the 
most highly specialized of all. — 
It is perhaps what we ought to expect, that the creek species should 
be more diverse and highly organized than any other fishes, for they 
must have had longer experience of fresh-water life. As the continent 
first began to rise from the sea, all its streams were necessarily small, 
and its first fishes were consequently those adjusted to life in creeks. 
As the process continued, as the surface of the land became more di- 
versified, as the small stream systems of the coast ate their way back, 
with many lateral branches, and united to form large rivers, and these 
again to make rivers of the largest size, new habitats would be formed, 
both in the uplands and along the coast, and new adaptations of fishes 
to them would naturally lead to about the kind of classifiable diversity 
which we actually find. 
That ecological differentiations and divisions among fresh-water 
fishes are, as a rule, of no very compelling force is shown in a remark- 
able manner by the fact that the whole system of such distinctions breaks 
down almost completely at least once a year, when a great migration 
movement up-stream and into shallow water seizes all species alike, under 
the overpowering impulse of the breeding instinct. At this time fishes 
of the most varied habit and habitat seem temporarily to desert or forget 
their favorite places of resort, and throng together, indifferent to their 
individual welfare, in search of places for the deposit of their eggs, 
and, with many species, for the subsequent care and protection of their 
young. Even under less extraordinary circumstances I have found, in 
fact, that a large river like the Illinois becomes a sort of metropolis of 
the fish population of its drainage basin, in which representatives of the 
various groups or associations, separate and distinct in its headwaters 
and smaller tributaries, may be found indiscriminately commingled, just 
as in this great city we see people from scores of smaller cities and 
hundreds of smaller towns and thousands of rural communities. Sunfish 
species, for example, which rarely occur in each other’s company in 
collections from the smaller tributaries, were found together twice as 
often in collections from the larger streams. 
