DARWIN-WALLACE CENTENARY— DE BEER 335 
South American type of rodent and not on that of North America or 
the Old World? 
The third set of observations was concerned with the fact that in 
the pampas he found fossil remains of large mammals covered with 
armor like that of the armadillos now living on that continent. Why 
were these extinct animals built on the same plan as those now living? 
On the view that species were immutable and had not changed since 
they were severally created, there was no rational answer to any of 
these questions, which would have had to remain as unfathomable 
mysteries. On the other hand, if species, like varieties, were subject 
to modification during descent and to divergence into different lines 
of descent, all these questions could be satisfactorily and simply an- 
swered. The finches of the Gal&pagos resemble each other and those 
of South America because they are descended from a common ancestor ; 
they differ from one another because they are each adapted to modes 
of life restricted to their own particular island, one, for instance, feed- 
ing on seeds on the ground and another on insects in trees. The vol- 
canic nature and physical conditions of the Galapagos Isiands re- 
semble those of the Cape Verde Islands, and yet the Galapagos birds 
all differ from the birds of the Cape Verde Islands: therefore it is 
not the physical conditions of the islands that determine their differ- 
ences. These differences arose because the Cape Verde Islands birds 
share a common ancestor with the birds of Africa, whereas the Gala- 
pagos birds share a common ancestor with those of South America. 
The hares of South America are built on the South American rodent 
plan because all South American rodents are descended from a com- 
mon ancestor. The fossil Glyptcdon resembles the living armadillos 
because they also share a common ancestor; this case is particularly 
important because, if living species show affinity with extinct species, 
there is no necessity to believe that extinct types of animals have left 
no living descendants. They may have representatives alive today, 
and this means that the whole wealth of the paleontological record 
of fossils is available as material for the study of the problem of 
evolution. 
In possession of a working hypothesis that species have undergone 
evolution and successive origination by descent, with modification, 
from ancestral species shared in common with other species, Darwin 
next proceeded to search the whole field of botanical and zoological 
knowledge for evidence bearing on his hypothesis. He realized that 
no general principle that explained the evolution of animals was ac- 
ceptable unless it also applied to plants. The result was one of the 
most remarkable attacks on a problem ever made by the inductive 
method of searching for facts, whatever their import might be. 
In the first place, in cultivated plants and domestic animals such 
as the dahlia, the potato, the pigeon, and the rabbit, a large number 
