188 Proceedings of the Royal Physical Society. 
fossiliferous strata were being deposited, or from the first 
appearance of life on our globe, there never has been at any 
one time a total extinction of the flora or fauna from the 
remote period till the present time; and when we find in our 
Coal-Measures the constant reappearance of certain well- 
known species, after each local extinction in higher and 
higher stages of strata in the same locality, wé are naturally 
led to conclude that, while they became extinct over these 
tracts, they must have continued flourishing in other parts 
of our Carboniferous sea, and that they spread from these 
spots into their old localities wherever the condition of the 
sea-bottom again became favourable to their growth 
and development.” - 
In a letter from the same writer, dated 26th October 1887, 
Dr Young further says: “There can be no doubt of the 
repeated occurrence in higher and higher horizons of many of 
the marine and fresh-water fornis of life found in our Scottish 
Coal-Measures. Since this paper” (that quoted above) “ was 
printed, I have been able to trace other forms besides 
Lingula, that range from the very lowest fossiliferous marine 
beds up into that of the Permian formation. - It is, there- 
fore, quite unsafe to take any one organism as characteristic 
of any special horizon, for closer investigation of the strata in 
any country is constantly proving their recurrence or occur- 
rence in higher or lower beds.” 
It is seen, therefore, that not only genera, but even 
species, of Mollusca, have persisted through a vast period 
of time in Carboniferous rocks, and, in some cases, through- 
out the whole of this great period, extending from the 
Calciferous Sandstone Series to the Upper Coal-Measures. 
We can easily understand how this has been brought about, 
for marine, or fresh-water animals, endowed with the power 
of locomotion, would. migrate to more favourable localities, 
whenever the conditions of the area in which they lived 
became unfavourable for their life and development. Asa 
rule, we may conclude that these changes unfavourable for 
their growth and development came on gradually, such as 
the sinking or elevation of the sea-bottom, or alterations in 
the nature and amount of sedimentation. There may, and 
