CH. IX] OBJECTIONS TO THE HYPOTHESIS 99 



facts to back it. It rests upon a large number of very clear and 

 definite figures, which are so consonant with one another that 

 they must be explained; they cannot be passed by as unimport- 

 ant, an)' more than can those upon which Mendel's Law is based. 

 Further than this, Age and Area has been used as the basis for 

 numerous predictions, all of which have proved to be correct. 

 Unless, therefore, some other hypothesis can be found to explain 

 the facts, and make the predictions, and that a mechanical 

 hypothesis, on account of the mechanical regularity of the 

 figures, Age and Area must be regarded as holding the field for the 

 present. 



Ecological factors svork at right angles to the age factor, to a 

 considerable extent, and on groups of allied species, taken over 

 a long time, their influence will then rarely be visible, as regards 

 total areas. The objections of the ecologists should, it seems to 

 me, largely disappear when they fully realise the meaning of 

 the careful provisos with which Age and Area is hedged about. 

 Not only are there those already considered (groups often species, 

 and allied species), but it is also pointed out that conditions 

 must remain reasonably constant. A serious change of con- 

 ditions is bound to make a great change in the dispersal rate of 

 the plants subject to it. If it only comes after the plant has 

 already spread into the neighbourhood affected by it, it will 

 probably make little difference, unless it reach the margin of 

 the area to which the plant has reached. Merely to exterminate 

 a plant in a portion of its range does not affect the total as 

 marked b}' the outlying stations. 



Further than this, it is expressly stated that great modifica- 

 tions may be introduced by barriers, including ecological changes, 

 changes of climate, and the like. All these provisos, taken to- 

 gether, seem to me to make sufficient allowance for any possible 

 ecological influences, and the fact remains, as just stated, that 

 the figures, which are incontrovertible, go to shoAv the great, 

 and indeed overwhelming, effect of mere age, when working 

 with a group of allied species over a long period. 



As has already been pointed out several times, age in itself 

 effects nothing, but the average result of the operation of 

 ecological and other factors is so uniform, when one works with 

 long periods, that the average rate of dispersal is also very 

 uniform. Barriers may of course completely stop it, but usually, 

 perhaps, only when they are physical, or due to such a cause as 



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