
 Boox V.] RELATIVE IMPORTANCE OF FOSSILS. 611 
favourably under the sea than on land, marine must be far more 
abundantly conserved than terrestrial organisms. This is true 
to-day, and has doubtless 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 
necessarily be gradation in importance, regulated chiefly by their 
‘possession of hard parts readily susceptible of preservation among 
marine deposits. Among the Protozoa, foraminifers, radiolarians, 
and sponges, possessing siliceous or calcareous organizations, have 
been preserved in deposits of all ages. Of the Ccelenterates those 
which, like the corals, secrete a calcareous skeleton are important 
rock-builders. The Echinoderms have been so abundantly preserved. 
that their geological history and development are better known 
than those of most other classes of invertebrates. The Annelides, on 
the other hand (except where they have been tubicolar), have almost 
entirely disappeared, though their former presence is often revealed 
by the trails they have left upon surfaces of sand and mud. Of all the 
marine tribes which live within the juxta-terrestrial belt of sedimen- 
tation, 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 mineralized 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 paleeontological 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 of 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. 
Of land animals the vast majority perish, and leave no permanent 
trace of their existence. Predatory and other forms whose remains 
may be looked for in caverns or peat-mosses, must occur more 
numerously in the fossil state than birds, and are correspondingly 
more valuable to the geologist for the comparison of different strata. 
Another character determines the relative importance of fossils as 
geological monuments. All organisms have not the same inherent 
capability of persistence. The longevity of an organic BS aha: on 
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