^«- ^i] AGE AND AREA 63 



endemic, figures which obviously agree with the rule. If one 

 take the figures in groups, one may safely say that the wides 

 will range the most, the endemics the least. 



In the same way, one must work, not only with groups of 

 species, but with groups of allied species, which will have more 

 or less the same dispersal-mechanisms and the same reactions 

 to their surroundings. If A, B, C be three species with wide 

 separation in relationship, and great differences between them 

 in regard to habit, dispersal-method, or other things, their rates 

 of dispersal may be entirely different, and A may travel ten 

 times as fast as C. But with a group of ten allied species one 

 will be fairly safe. 



Changes of condition, again, might evidently completely alter 

 the relative rates of dispersal of species, or might even stop 

 some of them altogether. And we must also take account of the 

 presence and action of barriers, already discussed, rememberino- 

 that some forms may cross a barrier when it has become quite 

 impassable to others. 



Age shows clearly in the distribution figures because it always 

 pulls the same way, whereas other causes of dispersal will either 

 tend to cancel one another by pulling different wavs, or more 

 commonly to exert a practically uniform pull upon' a group of 

 allied species, so that when two groups of allies are compared 

 one will be able to see the relative effects of age upon either' 

 In any single species its effects are liable to be completelv hidden 

 by those of some of the other causes, just as the effect of gravity 

 which is admittedly universal, is hidden in the case of an aero- 

 plane, a balloon, or a mo^-ing bullet. 



The most recent expression of the rule of Age and Area so 

 far pubUshed (133) is as follows: 



The area occupiedi at any given time, in any given country 

 by any group of allied species at least ten in inimber, depends 

 chiefiy, so long as conditions remain reasonablv constant, upon 

 the ages of the species of that group in that country, but mav 

 be enormously modified by the presence of barriers such as seas^ 

 rivers, mountains, changes of climate from one region to the 

 next, or other ecological boundaries, and the like, also by the 

 iietiuu of man, and by other causes. 



Extensions, Avhich will be considered below, have since been 

 given to Age and Area, which appears to be a general law cover- 

 ing all or nearly all the plants now existing upon the globe, and 

 1 Determined by the most outlying stations. 



