VIL. 
THE DISTRIBUTION OF SPECIES. 
In the present paper I shall consider certain facts 
of animal distribution as related to the origin of spe- 
: cies. There are many difficulties in 
eee aoe bringing the facts of geographical distri- 
bution down to the needs of concrete il- 
lustration. And in this connection it is especially im- 
portant to distinguish single illustrations from argu- 
ments. Isolated cases of geographical variations in 
species, for example, would not have great value as ar- 
guments for the development theory were the cases 
really isolated. The force lies in this fact, that these 
cases are typical; that what may be said of one is true 
of a thousand. In like manner the full force of the 
laws of homology and heredity can only be felt when 
their effect is cumulative, asin the mind of the anato- 
mist who has followed each organ through its protean 
disguises in a wide range of forms. Again, the force 
of the argument drawn from embryology does not come 
from a knowledge of the changes in a single egg. All 
these studies need the second premise, obtained by years 
of comparison in different fields of investigation, that 
no case is isolated. Without this premise, the argument 
would be incomplete. The few cases of development 
or change which can be brought to popular notice are 
simply illustrations and not proofs. 
14 IgI 
