FOSSILS 95 
obviously been subjected to much compression, fossils are 
usually highly distorted and often recognised with difficulty. 
By far the great majority of fossils are of marine origin, 
most of the sedimentary formations in which they occur 
having been deposited upon the floor of the sea. Fresh- 
water and terrestrial accumulations form an inconsiderable 
proportion of the series of stratified rocks, so that relics of 
the occupants of former rivers, lakes, and dry lands are of 
relatively infrequent occurrence. At the present day aquatic 
animals largely exceed terrestrial animals in number, and the 
Same was the case in earlier ages. On the other hand, in 
the world of to-day plants are mostly terrestrial forms, and 
although we know very little of the land-plants of the earlier 
geological periods, there is no reason to doubt that terrestrial 
floras have, for unnumbered ages, greatly surpassed marine 
floras in abundance and variety. The conditions for the 
accumulation and preservation of plants in the still waters 
of lakes, lagoons, and estuaries, are upon the whole more 
favourable than those that obtain upon the sea-floor. More- 
over, many seaweeds, with their loose, cellular tissues, are more 
readily decomposed than the great majority of the land- 
plants with their more enduring vascular tissues. For these 
and other reasons terrestrial plants occur in places more 
abundantly and in a better state of preservation than sea- 
weeds. Nevertheless, impressions and casts of the latter are 
not uncommon in strata of all ages, and hence it may be said 
that seaweeds are more widely distributed as fossils than 
land-plants. 
It is obvious, therefore, that from a general point of view, 
marine organic remains are of most importance to the student 
of historical geology. It is unquestionable that the records 
of past times are preserved chiefly in the marine formations 
Siete ‘earth’s crust: It is by studying these records that 
we are able to follow the main lines along which the world’s 
development has taken place. The histories revealed by 
freshwater and terrestrial accumulations are, as it were, only 
episodes, although these episodes are usually most interesting 
and instructive. Now and again, indeed, they may be said 
to constitute more or less complete chapters of the general 
world-history. They tell us of the life of the land, of which 
