354 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1958 
Nobody would have welcomed these developments of biological 
science more than Darwin himself, as a glance at the last few pages 
of “The Origin of Species” will show. It is therefore appropriate 
to return to the problem with which this article began. As is now 
certain, species are not immutable but have undergone change, and 
many examples have been given above. Evolution can take place 
up to a point without the production of new species, but if this process 
continues the time must come when new species originate, and it is 
legitimate to ask whether modern research has revealed any evidence 
of this. The answer is that new species can be seen originating in 
nature here and now, and new species have been artificially produced 
in the laboratory. 
Speciation takes place when, for various reasons, populations cease 
to breed with neighboring populations and, under different conditions 
of selection, accumulate heritable variations by mutation and recom- 
bination of genes in different directions. As E. Mayr has shown, 
some form of biological isolation between portions of populations is 
a necessary condition for divergence leading to the formation of new 
species and higher groups. 
Among the kinds of isolation that are chiefly responsible for the 
origination of species, geographical isolation is the most important; 
it involves physical barriers such as oceans, mountain ranges, or 
deserts which separate whole populations. Geographical races are 
the chief raw materials from which new species are formed, and it 
was the different finches on the different Galipagos Islands which 
first suggested to Darwin that evolution had occurred. Here, to 
various extents, geographical isolation has assisted the origination 
of a number of species. 
A ease in which geographical isolation may be expected to produce 
its effects at almost any moment now is provided by the gulls. These 
birds occupy a zone shaped like a ring around the North Pole and 
form what B. Rensch has called a chain of races. Starting with the 
British lesser black-backed gull, with its dark mantle and yellow legs, 
this is found to grade into the Scandinavian lesser black-backed gull, 
and, continuing in an easterly direction around the chain, this in 
turn grades into the Siberian Vega gull with its lighter mantle and 
dull flesh-colored legs. The Siberian gull grades into the American 
herring gull, which in turn grades into the British herring gull, with 
its light mantle and pinkish legs. Although the British lesser black- 
backed gull may be regarded as belonging to the same species as all 
the other gulls in the chain to the east of it, when it is compared 
with the other end of the chain represented by the British herring 
gull the twe may almost be regarded as separate species. Already 
they differ not only in color but in habits, for the latter nests on 
cliffs and is dispersive in winter, whereas the former breeds inland 
