648 On the Vertical Range of Certain Fossil Species _ [July, 
s. “One from Vandermark’s creek with large examples of S. 
disjuncta contains a ventral valve of S. mesocostalis. 
6. “ Another bearing Professor Hall’s ticket (279), from Cayuta 
creek, N. Y., has S. disjuncta with S. mesocostalis.” 
Professor Whitfield also adds : 
“ On one small specimen weighi ng ten or twelve ounces and 
bearing the label ‘ Cayuta creek,’ with characteristic specimens of 
S. mesocostalis and S. disjuncta, there is an imperfect imprint of 
the medio-dorsal part of a ventral valve presenting the features of 
S. mesostrialis as it occurs at some western localities [of N. Y.], 
but its outline is too imperfect to admit of positive identifi- 
cation. 
The testimony obtained from the collection in New York, 
therefore, agrees in every point with that given in Professor Wil- 
liam’s letter, and suggests in addition the possible association of 
all the three species at Cayuta creek. 
Yet one fact more may be given. Professor H. W. Geiger, of 
the U. S. Geological Survey, has informed the writer that he pos- 
sesses a slab from the Chemung of Virginia on which S, meso- 
strialis and S. disjuncta are lying side by side. 
In the face of these facts it is quite impossible any longer to 
maintain the sharp delimitation of the horizon of these three spe- 
cies above and below. It is evident that though characterizing, 
probably in some places by their abundance, certain zones, they 
are not by any means limited to these zones, but invade each 
other’s territory to an undefined extent. 
This result is more in harmony than its opposite with our pres- 
ent views of the progress of life on the globe. We have no rea- 
_ son to believe that species came suddenly into ascendancy and 
then as suddenly went out, especially at times unmarked by any 
catastrophe, as was the case in the Chemung era in Middle Penn- 
sylvania and New York. On the contrary, the general belief in 
evolution involves the special belief that every species has, in 
ordinary circumstances and barring accident, had its time of rising, 
culmination and decline, during which its life has overlapped the 
life of other kindred and perhaps derived species. Nothing is less 
likely, a priori, than that three species of spirifer should lie like 
three drawers in a geological cabinet one above another. Noth- 
_ ing is more likely than that each should occur in gradually in- 
-~ ¢reasing numbers until it reached its maximum, and then in grad- 
ad diminishing numbers until it died away. Each may then 
