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‘ 
_ Sept. 2, 1886] 
NATURE 
419 
identical or so closely allied as to be possibly varietal forms.’ In 
like manner the early plants of the Upper Silurian, Devonian, and 
Carboniferous present many identical species ; but this identity 
becomes less marked in the vegetation of the more modern times. 
In so far as plants are concerned, it is to be observed that 
the early forests were largely composed of cryptogamous plants, 
and the spores of these in modern times have proved capable 
of transmission for great distances. In considering this we 
cannot fail to conclude that the union of simple cryptogamous 
fructification with arboreal stems of high complexity, so well 
illustrated by Dr. Williamson, had a direct relation to the 
necessity for a rapid and wide distribution of these ancient 
trees. It seems also certain that some spores, as, for example, 
those of the Rhizocarps (see paper by the author on ‘‘ Paleozoic 
Rhizocarps,” Chrcago Transactions, 1886), a type of vegetation 
abundant in the Palzozoic, and certain kinds of seeds, as tho=e 
named -#¢heotesta and Pachytheca, were fitted for flotation. 
Further, the periods of Arctic warmth permitted the passage 
around the northern belt of many temperate species of plants, 
just as now happens with the Arctic flora ; and when these were 
dispersed by colder periods they marched southward along both 
sides of the sea on the mountain-chains. 
The same remark applies to northern forms of marine inverte- 
brates, which are much more widely distributed in longitude 
than those further south. The late Mr. Gwyn Jeffreys, in one 
of his latest communications to this Association, stated that 54 
per cent. of the shallow-water mollusks of New England and 
Canada are also European, and of the deep-sea forms 30 out 
of 35; these last of course enjoying greater facilities for migra- 
tion than those which have to travel slowly along the shallows 
of the coasts in order to cross the ocean and settle themselves on 
both sides. Many of these animals, like the common mussel 
and sand clam, are old settlers which came over in the Pleisto- 
cene period, or even earlier. Others, like the common peri- 
winkle, seem to have been slowly extending themselves in 
modern times, perhaps even by the agency of man. The older 
immigrants may possibly have taken advantage of lines of coast 
now submerged, or of warm periods, when they could creep 
around by the Arctic shores. Mr. Herbert Carpenter and other 
naturalists employed on the Challenge collections haye made 
similar statements respecting other marine invertebrates, as, for 
instance, the Echinoderms, of which the deep-sea crinoids 
present many common species, and my own collections 
prove that many of the shallow-water forms are common. 
Dall (‘‘ Report on Alaska”) and Whiteaves (Zvansactions 
R. S. C.) have shown that some mollusks and Echinoderms 
are common even to the Atlantic and Pacific coasts of 
North America; a remarkable fact, testifying at once to 
the fixity of these species and to the manner in which 
they have been able to take advantage of geographical changes. 
Some of the species of whelks common to the Gulf of St. 
Lawrence and the Pacific are animals which have no special 
locomotive powers even when young, but they are northern forms 
not proceeding far south, so that they may have passed through 
the Arctic seas. In this connection it is well to remark that 
many species of animals have powers of locomotion when young 
which they lose when adult, and that others may have special 
means of transit. I once found at Gaspé a specimen of the 
Pacific species of Coronula, or whale-barwacle, the C. regine of 
Darwin, attached to a whale taken in the Gulf of St. Lawrence, 
and which had probably succeeded in making that passage 
around the north of America which so many navigators have 
essayed in vain. 
It is to be remarked here that while many plants and marine 
invertebrates are common to the two sides of the Atlantic, it is 
different with land animals, and especially vertebrates. I do 
not know that any fossil insects or land-snails or millipedes of 
Europe and America are specifically identical, and of the 
numerous species of batrachians of the Carboniferous and reptiles 
of the Mesozoic all seem to be distinct on the two sides. The 
same appears to be the case with the Tertiary mammals, until in 
the later stages of that great period we find such genera as the 
horse, the camel, and the elephant appearing on the two sides 
of the Atlantic ; but eve: then the species seem different, except 
in the case of a few northern forms. 
1 See Davidson's ‘‘ Monographs on Brachiopods”; Etheridge, ‘*‘ Address 
to Geological Society of London”; Woodward, ‘Address to Geologists’ 
Association”; also Barrande’s ‘‘ Special Memoirs on the Brachiopods, 
Cephalopods, &c.”; Hall, ‘‘ Palaontology of New York”’; Billings, ‘* Re- 
ports on Canadian Fossils”; and Matthews, .‘‘Cambrian of New Bruns- 
wick,” Trans. R.S.C. 
Some of the longer-lived mollusks of the Atlantic furnish 
suggestions which remarkably illustrate the biological aspect of 
these questions. Our familiar friend the oyster is one of these. 
The first known oysters appear in the Carboniferous in Belgium 
and in the United States of America. In the Carboniferous and 
Permian they are few and small, and they do not culminate till 
the Cretaceous, in which there are no less than ninety-one so- 
called species in America alone ; but some of the largest known 
species are found in the Eocene. The oyster, though an 
inhabitant of shallow water, and very limitedly locomotive when 
young, has survived all the changes since the Carboniferous age, 
and has spread itself over the whole northern hemisphere 
(White, “‘ Report U.S. Geol. Survey, 1882-83 ”). 
I have collected fossil oysters in the Cretaceous clays of the 
coulées of Western Canada, in the Lias shales of England, in 
the Eocene and Cretaceous beds of the Alps, of Egypt, of the 
Red Sea coast, of Judzea, and the heights of Lebanon. Everywhere 
and in all formations they present forms which are so variable 
and yet so similar that one might suppose all the so-called 
species to be mere varieties. Did the oyster originate separately 
on the two sides of the Atlantic, or did it cross over so promptly 
that its appearance seems to be identical on the two sides? Are 
all the oysters of a common ancestry, or did the causes, what- 
ever they were, which introduced the oyster in the Carboniferous 
act over again in later periods? Who can tell? This is one 
of the cases where causation and development—the two scien- 
tific factors which constitute the basis of what is vaguely called 
evolution—cannot easily be isolated. I would recommend to 
those biologists who discuss these questions to addict themselves 
to the oyster. This familiar mollusk has successfully pursued 
its course and has overcome all its enemies, from the flat-toothed 
selachians of the Carboniferous, to the oyster-dredgers of the 
present day, has varied almost indefinitely, and yet has continued 
to be an oyster, unless indeed it may at certain portions of its 
cireer have temporarily assumed the disguise of a Gryphzea or 
an Exogyra. ‘The history of such an animal deserves to be 
traced with care, and much curious information respecting it 
will be found in the report which I have cited. 
But in these respects the oyster is merely an example of many 
forms. Similar considerations apply to all those Pliocene and 
Pleistocene mollusks which are found in the raised sea-bottoms 
of Norway and Scotland, on the top of Moel Tryfaen in Wales, 
and at similar great heights on the hills of America, many of 
which can be traced back to early Tertiary times, and can be 
found to have extended themselves all over the seas of the 
northern hemisphere. They apply in like manner to the ferns, 
the conifers, and the angiosperms, many of which we can now 
follow without even specific change to the Eocene and Creta- 
ceous. They all show that the forms of living things are more 
stable than the lands and seas in which they live. If we were 
to adopt some of the modern ideas of evolution, we might cut 
the Gordian knot by supposing that, as like causes can produce 
like effects, these types of life have originated more than once 
in geological time, and need not be genetically connected with 
each other. But while evolutionists repudiate such an applica- 
tion of their doctrine, however natural and rational, it would 
seem that nature still more strongly repudiates it, and will not 
allow us to assume more than one origin for one species. ‘Thus 
the great question of geographical distribution remains in all its 
force, and, by still another of our geological paradoxes, moun- 
tains become ephemeral things in comparison with the delicate 
herbage which covers them, and seas are in their present extent 
but of yesterday when compared with the minute and feeble 
organisms that creep on their sands or swim in their waters. 
The question remains, Has the Atlantic achieved its destiny 
and finished its course, or are there other changes in store for it 
in the future? The earth’s crust is now thicker and stronger 
than ever before, and its great ribs of crushed and folded rock 
are more firm and rigid than in any previous period. The 
stupendous volcanic phenomena manifested in Mesozoic and 
early Tertiary times along the borders of the Atlantic have 
apparently died out. These facts are in so far guarantees of 
permanence. On the other hand, it is known that movements 
of elevation along with local depression are in progress in the 
Arctic regions, and a great weight of new sediment is being 
deposited along the borders of the Atlantic, especially on its 
western side, and this is not improbably connected with the 
earthquake shocks and slight movements of depression which 
have occurred in North America. It is possible that these 
slow and secular movements may go on uninterruptedly until 
