292 MORPHOLOGY . 
Isolation. The importance of isolation in the formation of species 
is variously estimated, but that it is at least of great assistance seems 
evident to those best acquainted with species in nature. If a group of 
individuals possessing a certain variation were associated with a larger 
number of closely related individuals not possessing it, the intercrossing 
of the two groups might obliterate the distinction. On the other hand, 
if the varying group were isolated from all of its near relatives, so that 
there could be no intercrossing, the variation would be far more likely 
to persist and increase. In other words, a variation that otherwise 
might disappear may be established by isolation. 
The term isolation usually suggests geographical or topographical 
isolation, which is perhaps the most effective kind. Migration dis- 
tributes individuals widely, and the various barriers that segregate them 
into distinct groups are well known and need not be enumerated. The 
general tendency to dispersal inevitably leads to more or less isolation, 
and it seems probable to many that most species have been finally estab- 
lished in this way. In any event, it is evident to those familiar with the 
geographic or topographic position of species in reference to one another 
that this kind of isolation is a factor of very great importance in their 
determination. It does not produce them, but it gives them an oppor- 
tunity. 
There is also recognized what is called biologic isolation, which means 
that such variations may occur among closely related individuals that, 
although they may be associated in one habitat, they become incapable 
of crossing. This may result from a difference in the season for fertili- 
zation, in some structure that prevents crossing, or in various other ways. 
At present, this kind of isolation does not stand out as a factor in the 
determination of species so distinct and effective as does geographic 
isolation. 
Mendel's law. It is evident that whether new species arise by the 
cumulative results of natural selection acting upon small variations, or 
by the occasional sudden appearance of wide variations, a still more 
fundamental problem is to explain variation, which is one of the features 
of heredity. The study of heredity, therefore, which is fundamental to 
all evolutionary doctrine, is being prosecuted to-day with remarkable 
vigor. Conspicuous among the recently developed doctrines of heredity 
is Mendel's law, so called because it was first announced by Gregor 
Mendel, an Austrian monk. Mendel's publication of fifty years ago fell 
on sterile ground and passed into oblivion, until it was brought to light 
