NOVEMBER, 1905. OECOLOGICAL FEATURES OF EVOLUTION. 
173 
could produce any required change, for we know that many forms 
in profiting by unoccupied groups of contacts have developed a 
specialization from which they could never retreat, for so narrow 
is the line between hospitality and hostility that any change in 
their environment will render it violently hostile, even to extinc- 
tion. 
Changes in the environment that would produce a hostility of 
the second type described above (climate, surface, etc.) would be 
less liable to produce extinction and more liable to result in the 
survival of slightly changed forms than changes of the first type 
(other animals). As will be shown below slow changes are ample 
to account for the process described as mutation by Scott and 
''definite variation" by Osborn. 
Environmental hospitality as defined above is the condition 
when the sum of any animals contacts with its environment is 
favorable. It may be discussed under the same two heads as 
environmental hostility. 1. Contacts with other animals. 2. 
Other contacts. 
The first of these may be reduced to nil. By environmental 
isolation would be expressed the fact that the new form would 
not come into contact, either actively or passively, with other 
animals. It is obvious that this condition might occur in regions 
that were crowded with other animals if the habits or structure of 
the form permitted it to seize on a group of contacts that were 
not occupied by some other form, and if it was not an acceptable 
prey to other animals. This conception is a most useful one as 
it explains the possibility of the introduction of new forms and 
their development, even luxuriant development, in regions that 
are seemingly already crowded. 
Environmental isolation may be found immediately by 
an animal entering a new region or it may be achieved 
after a period of hostility; after a period of successful 
competition with other forms in passive hostility, or after a 
period of active hostility when the predatory form has become 
for some reason reduced, or by the development of new habits 
or structures which would render it independent of the other 
forms. (Environmental isolation might exist in envronmental 
hostility but it would be rare as it is such a powerful factor on 
the positive side of environmental hospitality). 
There is no thinkable isolation from contacts other than 
animal. 
Environmental hospitality once attained the perpetuation 
and increase of the animal would be assured. But the very in- 
