1886. | some of its Apparent Causes. 39 
the close of the secondary period, has been wonderfully sud- 
den.” 
In the same manner the trilobites as well as the Eurypterida 
ceased to exist at the end of the Palæozoic age; the Silurian 
graptolites disappeared with comparative suddenness ; the crinoids 
and brachiopods mostly, and the dinosaurs and ornithosaurs, aS 
well as pythonomorphs wholly perished during the Cretaceous 
period. 
The views we have presented, while opposed to ultra-uniformi- 
tarian ideas, have nothing in common with the Cuvierian catas- 
trophic doctrine of sudden, wholesale extinctions and re-creations. 
But known facts in paleontology postulate long periods of quiet 
preparation, succeeded by more or less sudden crises, or radical 
changes in the physical structure of continents, resulting in catas- 
trophies, both local and general, to certain faunas or groups of 
animals, as well as individual species. 
The biological changes were not due to climatic and geological 
changes alone, but it should be borne in mind that the great 
changes, slowly induced but not without striking final results, end- 
ing in the addition or loss of vast areas of land, induced exténsive 
migrations, the incursions of prepotent types which exterminated 
the weaker. The reaction of one type of life upon another, the 
results of natural selection, were apparent all through; but these 
secondary factors were active both during periods of quiet and 
periods of change. Here again we may quote from Darwin, the 
leader, next to Lyell, of the uniformitarian school, who remarks : 
“ We have every reason to believe, from the study of the Tertiary 
formations, that species and groups of species gradually disappear, 
one after another, first from one spot, then from another, and 
finally from the world. In some few cases, however, as by the 
breaking of an isthmus and the consequent irruption of a multi- 
tude of new inhabitants into an adjoining sea, or by the final sub- 
sidence of an island, the process of extinction may have been 
rapid.” 
Local extinctions due to local changes of level; the formation 
of deserts, saline wastes and volcanic eruptions and vast outpour-' 
ings of lava, such as took place in Oregon and Idaho during the 
Tertiary, with sub-marine earthquakes causing the death of fishes 
on a vast scale, these are quite subordinate factors, 
In closing this meager sketch of a subject which has not re- 
