128 
BULLpyriN OF THE UNITED STATES FISH COMMISSION. 
We learn from tliese tables that in all families bnt the Cyprinodontidct; with more 
than one species on the Pacific slope the extent of variation is greater than in the 
same families on the Atlantic slo])e.* This might have been expected if the number 
of siiecies were greater on the Pacific than on the Atlantic slope, bnt in most cases 
the reverse is true, as may be seen from the following table: 
Family or subfamily. 
Pacific slope. 
Atlantic slope. 
Genera. 
Species. 
Geuera. 
Species. 
Aciiien.seridie 
1 
2 
2 
4 
Catostominai 
4 
21 
4 
11 
Cvprinida' 
17 
75 
21 
175 
SahnoBitbe 
3 
12 
4 
12 
Cyiu'iuodontida^ 
4 
5 
9 
About 45 
Oa-sterosteifUB 
2 
4 
4 
7 
Ceiitrarchifbne 
1 
1 
9 
26 
Ooftidae 
1 
8 
1 
12 
I can conceive of bnt three possilile explanations for this variation: 
(1) The Pacific slope fauna maybe new as compared with the Atlantic. The 
comparatively new conditions may have thrown the characters into a condition of 
nnstable equilibrium with the selection of the adapted forms. The flnctuations in the 
fin rays of some of the species wonld lend weight to such a supposition. 
(2) The Pacific slo})e fauna may be of diverse origin. 
(3) Both of these factors may have contributed to bring about the present condition. 
This last seems to me to be the true solution. Most of the forms have undoubt- 
edly been derived within comparatively recent time from the Atlantic slope of North 
America, Avhile others have a decidedly Asiatic cast. 
Acipenser and Oncoidi.i/iich.HS are certainly of Asiatic origin. While I am not suffi- 
ciently acquainted with Asiatic minnows to speak with certainty, some of the genera 
of minnows seem to have a decided Asiatic affinity. 
Many of the Catostomincc, the Giiprinida’, and Salpelinns, Archoplites, and probably 
Cottus have all been derived from forms from the eastern slope of North America. 
* The ouly other exception is introduced by Thymallus. 
