Vice-President’s Address. 185 
been true in all past geological time. Hence, for the purposes 
of the geologist, fossil remains of marine forms of life far 
surpass all others in value. Among them there will neces- 
sarily be gradations in importance, regulated chiefly by their 
possession of hard parts, readily susceptible of preservation 
among marine deposits.” 
“Of all the marine tribes which live within the 
juxta-terrestrial belt of sedimentation, unquestionably the 
Mollusca stand in the front rank as regards their aptitude for 
becoming fossils. In the first place, they almost all possess 
a hard, durable shell, composed chiefly of mineral matter, 
capable of resisting considerable abrasion, and readily passing 
into a mineralised condition. In the next place, they are 
extremely abundant both as to individuals and genera. They 
occur on the shore up to high-water mark, and range thence 
down into the abysses. Moreover, they appear to have 
possessed these qualifications from early geological times. 
In the marine Mollusca, therefore, we have a common ground 
of comparison between the stratified formations of different 
periods. They have been styled the alphabet of paleeonto- 
logical inquiry. It will be seen as we proceed, how much, 
in the interpretation of geological history, depends upon the 
testimony of sea-shells. 
“Turning next to the organisms of the land, we perceive 
that the abundant terrestrial flora has a comparatively small 
chance of being well represented in a fossil state; that indeed, 
as a rule, only that portion of it which the leaves, twigs, 
flowers, fruits, or trunks are blown into lakes, or swept down 
by rivers, is likely to be partially preserved. Terrestrial 
plants, therefore, occur in comparative rarity among stratified 
rocks, and furnish in consequence only limited means of 
comparison between the formations of different ages and 
countries.” 
“ Another character determines the relative im- 
portance of fossils as geological monuments. All organisms 
have not the same -inherent capacity of persistence. The 
longevity of an organic type has, on the whole, been in 
inverse proportion to its perfection. The more complex its 
structure, the more susceptible has it been of change, and 
