SIMI ITS SHOWS SINGIDIEIN INS: 495 
Cuboides fauna is exotic, appearing. suddenly in the Tully lime- 
stone at the base of the Upper Devonian; but in Europe the 
Cuboides fauna belongs to a genetic series, and is autochthonous. 
The Tyvopites fauna of the Karnic stage, Upper Trias, is 
heterochthonous both in Europe, Asia and America, but the 
place where this fauna belongs to a genetic series is still 
unknown. 
In the same way several of the faunas of the Jura and 
Cretaceous in California appear to be heterochthonous ; some of 
these are known to be endemic in certain regions, others, on the 
contrary, are not. 
CONCLUSION. 
From the foregoing facts it will be seen that migration 
increases the struggle for existence by introducing forms into 
new conditions, to which they must adapt themselves, and that 
it also breaks up the established equilibrium and introduces new 
rivals, thus affecting autochthonous forms. Migration therefore 
works hand in hand with natural selection in killing off weaker 
forms, and changing the plastic ones by adaptation. 
It is also clear that migration strengthens the geologic 
evidence of great changes in physical geography in past time. 
Regions that now are continents have certainly been seas, and 
seas have probably been continents. 
Even the ocean abysses need not have been permanent, for 
their faunas have a modern aspect, and therefore must have 
changed from the old types. But from what we have seen of 
survivals of faunas under favorable conditions, we infer that 
without changes in conditions there is no reason why they 
should have changed. The natural inference then is that neither 
continental plateaux nor oceanic abysses have been permanent. 
JAMES PERRIN SMITH. 
STANFORD UNIVERSITY, 
California, May, 1895. 
