6 Our Rocks and their Fossils. 
marl, lias, oolite, greensand, chalk, eocene, miccene, pliocene, and 
drift—we can only give a single eon to be divided between them. 
Such facts will sufficiently suggest how comparatively modern are 
all these rocks when viewed “by the light of an absolute chrono- 
logy. Now, the first fishes do not occur till the Silurian—that is 
to say, in or about the seventh zon after the beginning of geolo- 
gical time. ‘The first mammals are found in the trias at the 
beginning of the tenth zon. And the first-known bird only 
makes its appearance in the oolite, about half-way through that 
latest period. ‘This will show that there was plenty of time for 
their development in the earlier ages. True, we must reckon the 
interval between ourselves and the date of the blue mud at many 
millions of years; but then we must reckon the interval between 
the lias and the earliest Cambrian strata at some six times as 
much, and between the las and the lowest Laurentian beds at 
nearly ten times as much. Just the same sort of lessening per- 
spective exists in geology as in ordinary history. Most people 
look upon the age before the Norman Conquest as a mere brief 
episode of the English annals; yet six whole centuries elapsed 
between the landing of the real or mythical Hengst at Ebbsfleet 
and the landing of William the Conqueror at Hastings; while 
under eight centuries elapsed between the time of William the 
Conqueror and the accession of Queen Victoria. But, just as 
most English histories give far more space to the three centuries 
since Elizabeth than to the eleven centuries which preceded them, 
so most books on geology give far more space to the single zon 
(embracing the secondary and tertiary periods) which comes 
nearest our own time, than to the nine eons which spread from 
the Laurentian to the Carboniferous epoch. ~In the earliest 
period, records either geological or historical are wholly wanting ; 
in the later periods they become both more numerous and more 
varied in proportion as they approach nearer and nearer to our 
own time. 
So, too, in the days when Mr. Darwin first took away the 
breath of scientific Europe by his startling theories, it used confi- 
dently to be said that geology had shown us no intermediate form 
between species and species. Even at the time when this asser- 
tion was originally made it was quite untenable. All early geolo- 
gical forms, of whatever race, belong to what we foolishly call 
“generalised ” types. For example, the earliest ancestral horse is 
partly a horse and partly a tapir: we may regard him as a ¢ertoum 
guid,a middle term, from which the horse has varied in one direction 
and the tapir in another, each of them exaggerating certain special 
peculiarities of the common ancestor and losing others, in accord- 
ance with the circumstances in which they have been placed. 
Science is now perpetually discovering intermediate forms, many 
