Vice-President’s Address. 189 
apparently have been, sudden extinctions of marine life in 
certain areas, as in the Upper Old Red Sandstone at Dura 
Den, where the fish have been overwhelmed and killed in 
shoals, but such cases would, probably, be rare, and I think we 
are warranted in concluding that marine organisms were able, 
in the great majority of cases, where the physical conditions 
necessary for their growth altered, to preserve themselves by 
migration. The proof of this is seen in the repeated recur- 
rence of the same organism in higher and higher horizons— 
and it is this very capability of migration and self-preserva- 
tion of the species through vast periods of time, which make 
the Mollusca, and marine organisms generally, so much more 
unsuitable than plants for determining horizons in the Carboni- 
JSerous formation. 
If we examine the conditions under which land plants 
grow, we see how unfavourable their conditions are, when 
compared with those of marine animals, for the preservation 
of their existence when any physical change took place which 
was unsuited to the conditions necessary for their hfe. Plants 
being fixed to the ground on which they live, subsidence, 
to even a slight extent, would be sufficient to cause their 
destruction and subsequent envelopment in mud and silt, as 
itis highly probable that they grew mostly on flat marshy 
tracts little elevated above the sea-level. 
Even elevation of the land would, probably, be little less 
fatal to their existence, by bringing about a drier situation 
with a probable less humid atmosphere. Of course it is quite 
unlikely that a complete extinction took place over large 
areas, but notwithstanding we fully admit this, there still 
remains the cardinal fact that plants are provided, only to a 
limited extent, with the means of preventing the evil con- 
sequences of a change of physical conditions, when compared 
with the advantages for preservation of their life possessed 
by organisms endowed with the power of locomotion. Some 
species of fossil plants persisted much longer than others, 
but even in the case of those which persisted longest, their 
range in time is not nearly so great as that of their con- 
temporary Molluscs. 
If we divide the Carboniferous formation into Upper and 
