THE DISTRIBUTION OF SPECIES. 219 
with time. They change with space because, as they 
move over the globe, with distance comes barriers, with 
barriers isolation, and any degree of isolation brings 
some change of conditions. This means, sooner or later, 
a distinction of species. 
Species change with time, because time brings changes 
in conditions. Epoch-making events of one sort or an- 
other come in to break even the most 
monotonous existence. If time could 
flow on evenly there need arise no 
change in life, adaptation being once 
established. If space were absolutely uniform without 
barriers or variation of conditions, life would flow on 
as uniformly. Where there is most monotony in condi- 
tions, as in the depths of the sea, there is least change 
in life, least formation of new species, and least tendency 
to progress through natural selection. 
Similar to geographical isolation in its nature and 
effects is physiologicalisolation. This appears in the de- 
velopment, in isolated races, of antipa- 
thies which serve as barriers to prevent 
the interbreeding with allied races or 
species. This condition among animals is the homo- 
logue of race hostility among men. Such a feeling of 
mutual aversion, whether accompanied by anatomical 
distinctions or not, must be at times a strong factor in 
the differentiation of species. 
The study of the problems of geographical distribu- 
tion is possible only on the theory of the derivation of 
species.. If we view all animals and 
plants as the results of special creations 
in the regions assigned to them, we have, 
instead of laws, only a jumble of arbitrary and meaning- 
less facts. We have been too fully accustomed to the 
recognition of law to believe that any facts are arbitrary 
How species 
change with 
time. 
Physiological 
isolation. 
Meaning of facts 
of distribution. 
