358 
NATURE 
[ Feb. 3, 1870 
gradual migration along the coast could they well double the 
end of the chain near the mouths of the Atrato and Magdalena, 
and so pass to the eastern side ; for the Sierra Nevada bars the 
way. 
Tea I believe, would offer better data in discussing this 
question of barriers than almost any other group of land animals, 
or than plants; they are more limited in range than the species 
of birds, afford a much larger body of facts than reptiles, mam- 
mals and shells, and are not so much subject to accidental means 
of transportation as plants. But although many Entomological 
collectors have visited Guayaquil and the Cordillera, we have no 
published lists and no authentic information about localities. 
Mr. Buckley’s journey offers us, then, the chance of obtaining 
the details so much required, since he collected assiduously all the 
way up from the level of the sea to the edge of the snow, and 
the same conversely on the opposite side, writing the locality on 
the envelope of every specimen. 
Tam inclined to think that the efficacy of physical or geographi- 
cal barriers in limiting the distribution of animals and plants has 
been much over-estimated, and that this circumstance has vitiated 
much of the reasoning that has been employed in discussing 
various difficult problems in Natural History. By physical bar- 
riers, of course, are meant barriers of the inorganic world, such 
as a continuous mountain-range with regard to species of the 
plains and, conversely, a continuous plain with regard to species 
of the mountains (e. g. Parnassius, Erebia, Oreina, Nebria, &c.) 
The sea is thus a barrier to land-species, a water-shed to fresh- 
water species, a continuous tract of forest to species of the 
savannah or steppe, and so on. Barriers of the organic world, 
which of course are ‘‘ physical”’ also, are quite a different set of 
agencies. They are the hindrances offered to the dissemination 
of a species by other species already in full possession of the 
domain and well adjusted to its conditions by constitution and 
habits. To this may be added the limitations to distribution 
observable without any physical obstacle being perceptible. 
There are certain classes of facts which seem to me to indicate 
that these less obvious kinds of barrier are far more effective 
than those more imposing ones of mountain, desert, sea, and 
so forth. 
One set of these facts is exemplified by the well-known case of 
distribution of insects between the east and west in the southern 
part of our own island. I am not aware that comparative 
lists have yet been published; but it will not be disputed 
that many hundreds of species of Coleoptera, for instance, 
are known in the east, many of them abundant, which are 
totally unknown in the west, and a smaller number are 
known in the west which are not found in the east. In 
In cases like these a difference of climate may be the cause of 
the limitation. But there is another set of facts requiring quite a 
different explanation : this is the limited ranges of closely-allied 
species in the plains of Tropical America. I have already 
elsewhere recorded the fact that, in the forest plains of the 
Amazons, where there are no differences of level worth mentioning, 
and no physical barriers, the species of a large number of 
genera are changed from one locality to another, not more than 
200 or 300 miles apart. This is most distinctly marked on the 
Upper Amazons, where the country may be mapped out into 
areas of a few hundred square miles each, every one containing 
numerous species of such genera as Ithomia, Melinzea, Eubagis, 
Doryphora, Erotylus, &c., &c., allied to but quite distinct from 
their representatives in the others. From what I have seen of 
Mr. Buckley’s collections on the eastern side of the Andes, I think 
the same limitation of areas must occur there also; and judging 
from the few species I know as coming undoubtedly from the 
Guayaquil side of the Cordillera, the butterfly faunas of these areas 
in the uniform country of the east are pretty nearly as distinct from 
each other as the species east of the Andes are distinct from those 
west of the mountains. We here again feel the want of facts, 
such as Mr. Buckley collected, but which have not yet been 
published, to teach us exactly what species are found east and 
what west of the mountains, and how the great multitude of 
closely-allied species are distributed in the narrow tract explored 
on the east. My own observations in the level plain, a few 
hundred miles further east, show distinctly, however, that the 
most effective possible barriers are there opposed to the spread of 
hosts of species without any physical barrier which is perceptible 
by our senses. The explanation of the fact, I believe, is this, 
that there really are subtle differences of physical conditions 
from place to place, even in a uniform region; slight dif- 
ferences in soil, humidity, succulence of foliage, and so forth, 
which require in each a re-adjustment of the constitution 
of any new immigrants from adjoining areas; but that each 
area being kept well stocked with allied species already 
adjusted to its minute conditions, such migration rarely occurs. 
Thus a limit is put to the spread of species by species them- 
selves, which produces similar results on the actual distribu- 
tion of forms throughout the world, to those produced by mighty 
physical barriers such as the Andes, 
There is yet one other consideration remaining. If these 
barriers are not required to explain the limitation of faunas, it 
does not follow that they do not act as barriers all the same; 
but it is, I think, difficult to prove it. If 1,200 miles of sea do 
not form a sufficient barrier against the stocking of the Azores 
with insects from Western Europe, I do not think sixty miles of 
mountain should be assumed to prevent for tens of thousands of 
years, the transport of species, in the egg state, by birds or 
currents of air, from one side to the other. I may add, in 
conclusion, that if the efficacy of barriers of this nature has 
been overrated, some important conclusions regarding changes 
on the earth’s surface will have to be reconsidered ; such, for 
instance, as that of the extension of a glacial epoch over nearly 
the whole earth—a hypothesis conceived by Darwin to explain 
the existence of the same genera and sometimes the some species 
in high latitudes, both in the northern and southern hemisphere, 
whilst absent from the intervening zones. I believe that, with 
some very obvious exceptions, such as Mammals and Batrachians, 
there can be no limit placed as to the dissemination of a 
species, provided there are unoccupied areas suitable to it, in 
any part of the earth, and provided also time sufficient be 
allowed for the process. 
THE GRESHAM LECTURES 
WO lectures were delivered in Gresham College on the 
evenings of the 14th and 15th January, by Dr. Symes 
Thompson, the Gresham Professor of Medicine. 
The first of these lectures embraced a theme admirably adapted 
to fulfil the popular object with which this City professorship has 
been established, and as eminently suited to the present season: 
it treated of ‘* Catching Cold.” 
The Professor first described, by reference to large drawings, 
the structure and arrangement of the parts concerned in the dis- 
order—laying open the arcana of the nose, frontal sinuses, throat, 
voice-box, and chest, and showing the intimate relations by which 
these parts are connected, and the way in which modern science 
has found means to bring their inmost recesses under observation, 
and contrasting the precise knowledge of the present period with 
the ante-Schneider days, when all catarrhal defluxions were held 
to be outpourings of the brain. It was demonstrated that the 
ordinary cold is simply, in the first instance, congestion of the 
warm, moist, blood-charged membrane, which lines all these 
cavities and is continuous throughout the series of them; 
but that this congestion is apt to pass on, under unfavourable 
circumstances, to inflammation, and to consequent derangement 
of structure. The congestion merely means that more blood is 
thrust upon, and retained in, the minute channels and vessels of 
the membrane, than those channels and vessels can healthily 
accommodate The first cause of this forced engorgement is 
that cold is extensively applied to the internal skin, which then, 
under the constringing and contracting influence, drives its own 
blood out, partly into these surcharged tracts of mucous mem- 
brane. The injurious effect known as “ cold” is now sure to be 
realised if this external chill is experienced when the general 
system is weakened by exhaustion. It is also, in some persons, 
more apt to be produced at certain regular periods. 
The prevention of colds is to be accomplished by keeping the 
skin in a healthy and vigorous state, so that it may at once 
resume its proper and normal condition when chills have been 
suddenly applied to it: then the internal congestions are avoided 
or removed simultaneously with the external contraction and 
stagnation. The habitual use of cold bathing in the early morning 
is one very powerful means to this end: it trains the vessels of 
the skin to rise vigorously into renewed action after the applica- 
tion of a chill. The relaxing influence of over-heated apartments 
should be avoided, because that saps the power of vigorous 
reaction ; but, in cold weather, the utmost care should be taken 
to have the entire skin efficiently protected by warm clothing. 
The powers of the system in periods prone to the production of 
colds, and most especially when the temperature of the external 
