PASSAGES FKOM POPULAR LECTURES. 
77 
The earliest sedimentary rocks appear to have been deep 
marine deposits, in which no traces of land plants could be 
expected ; but on reaching the Devonian beds we come upon 
swamp and shore formations, and here at once the fern-forms 
show themselves. 
As to what the world was like before the Laurentian 
epoch we have as yet no knowledge. It may be that if ever 
the beds of existing oceans become inhabited continents, and 
the present continents the beds of future oceans, other sedi¬ 
mentary deposits will be brought to view still older than the 
oldest with which we are now acquainted, and will unfold to 
future explorers the secrets of the dawn of life for which we 
seek so eagerly and yet so fruitlessly. Or it may be that all 
those early strata have been melted by internal heat into the 
granites which seem to underly the fossiliferous strata every¬ 
where, and that their priceless records have thus become 
unreadable and lost for ever. We only know that the earliest 
traces we have vet found of land vegetation are the remains of 
ferns and lycopods, and that there is reason to believe that 
out of some original variations in the development of some 
fern-forms, and by a succession of such variations, perhaps at 
long intervals, during the unknown ages of the past, there grew 
up gradually the arborescent ferns, the palms, the bamboos, 
the grasses, the rushes, and finally the beautiful flowering 
monocotyledons—lilies, irises, amaryllids, and orchids, and 
that out of some similar variations of the lycopod-form were 
gradually moulded the tall conifers or fir trees, the broad¬ 
leaved oaks, and poplars and sycamores, and at last those 
flowering dicotyledons, the roses, magnolias, laburnums, lilacs, 
and all the host which make the glory of our present summers. 
But through all these changes the original types have been 
preserved even to this day. Some individuals may have 
varied widely, but in every generation some have come true 
to the leading features of the type. The lycopods of this age 
are clearly of one order with the lycopods of the coal, and the 
ferns which grow in our woods are, in essential points of 
structure, one with the ferns of all ages, though in non- 
essentials, in points which distinguish species and genera but 
not orders, the changes are marked and numerous. 
At the present time there are known to botanists about 
2,600 distinct species of existing ferns. The majority of these 
inhabit tropical regions and moist situations. Mountainous 
islands, shaded by woods and surrounded by warm seas and 
moist atmosphere, are the chosen homes of the fern family. 
Outside of the tropics the two islands in the northern and 
southern temperate zones which are richest in fern species for 
