DARWIN-WALLACE CENTENARY—bDE BEER 355 
on moors and is migratory in winter. If at any time the chain be- 
comes severed by the erection of a sterility barrier at any point, either 
through inability to breed or through a rupture of the chain by local 
extinction of the gull population, the two British gulls will effectively 
have originated new species. 
Geographical isolation is important for the origin of species of 
plants as well as of animals, but there is another form of isolation 
which appears to be restricted to plants and involves the sudden erec- 
tion of sterility barriers between individuals in the same population 
as a result of changes in the chromosome mechanism. This is known 
as genetic isolation. When Primula verticillata is crossed with Pri- 
mula floribunda, hybrid offspring are produced, but they are sterile 
because the chromosomes of one parent species are incompatible 
with those of the other, and the intricate machinery involved in the 
formation of germ cells is thrown out of gear. Occasionally, how- 
ever, the hybrid plant undergoes doubling of its chromosomes, a 
condition known as polyploidy, and when that has occurred the 
hybrid is able to breed with hybrids similar to itself because all the 
chromosomes have compatible partners, but it is sterile in respect 
to both parent species. Furthermore, the hybrid is not only true 
breeding but is different in structure and in habit from each of its 
parent species. It therefore fulfills all the criteria of a species and 
has been called Primula kewensis. Many other new species have 
originated by intentional hybridization and accidental polyploidy in 
this way. Some of these artificially produced species have been found 
to be identical with, and to breed with, wild species, and this is the 
proof that this method of species formation occurs in nature. 
THE CENTENARY OF EVOLUTION BY NATURAL SELECTION 
In conclusion, it may be said that during the hundred years that 
have elapsed since Darwin and Wallace first published their theory, 
the fact of organic evolution is now universally accepted and its 
mechanism has been formally explained. 
The alternative to evolution is so naive that it comes as a shock to 
realize that as recently as one hundred years ago, ideas such as called 
for the following questions could still be current: “Do they really 
believe that at innumerable periods in the earth’s history certain 
elemental atoms have been commanded suddenly to flash into living 
tissues? Do they believe that at each supposed act of creation one 
individual or many were produced? Were all the infinitely numerous 
kinds of animals and plants created as eggs or seeds, or as full 
grown? And in the case of mammals, were they created bearing 
the false marks of nourishment from the mother’s womb?” Darwin 
might well allow himself to ask these questions, for he and Wallace 
had found the answer to them. 
