Fig. 2.— Hardware cloth traps used to collect parent species from central Illinois lakes and ponds. These traps were used 
also to sample hybrid populations. A, traps are shown here being assembled. B, a trap is being set in a small gravel pit pond. 
Fig. 3. — Procedures used in making laboratory crosses. A, eggs from a female red-ear sunfish are being stripped into a 
damp petri dish, where they are immediately fertilized with sperm from a male bluegill. B, after the fertilized eggs have been 
allowed to set for 2 minutes, they are placed in other petri dishes; they adhere to the bottoms of the dishes. 
Field Crosses. — Each of 16 small ponds (0.02 to 
0.9 acre) containing no fish was stocked with sexually 
mature fish: females of one species and males of an- 
other, table 2. At frequent intervals, the ponds were 
checked, and any nests that were found were examined 
to see if they contained eggs or fry. The ponds were 
also seined and, when young F] hybrids were present, 
some of the hybrids were removed and used for stocking 
other ponds. Fy hybrid sunfishes produced by field 
crosses, like those produced by laboratory crosses, 
were held in ponds for one or more growing seasons. 
RESULTS OF HYBRIDIZATION 
In this paper, direct genetic isolation refers to spe- 
cies isolation resulting from incompatibility between 
the gametes of different species, while indirect genetic 
isolation refers to species isolationresulting from such 
differences as time of spawning, habitat requirements 
for spawning, and mating behavior patterns. 
Laboratory Crosses. — Several thousand free-swim- 
ming Fy, hybrids were produced in the laboratory from 
each of the six possible P] crosses; these hybrids 
