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 



