( cxsiii ) 



relatively simple cases, like Darwin's famous example of the 

 way in which the fertility of red clover may depend, through 

 the agency of humble-bees and field-mice, on cats. To supply 

 such proof in general would seem as difficult as it would be to 

 ti'aee the paths of the particles in a storm-tossed sea, which, 

 while in perpetual movement under the influence of the flowing 

 and ebbing tides, and of the waves raised by the gusts of wind 

 that sweep over its surface, aided by the fluctuating contribu- 

 tions from the river-mouths and rare upheavals from the 

 " abysmal depths," yet maintains an equable general level ; even 

 more difficult, because the influences that vary the face of 

 organic nature are infinitely more complex than those which 

 perturb the surface of the stormiest sea. 



Great, however, as are the difficulties of proof, that is no 

 reason why as much evidence as may be should not be obtained, 

 thus conforming, so far as possible, to the demand — always a 

 reasonable one — for evidence to support inference.* And the 

 endeavour to obtain it, however imperfect the result, cannot 

 fail to supply a fund of interesting and useful knowledge. 



Persisteiit variability. 



Let us now give some attention to the persistence of vari- 

 ability in the forms of established species. Looking to the exces- 

 sive keenness of the competition that exists for vacant places, 

 it does not seem easy to realise how some one of the " merest 

 trifles " before adverted to can fail to give an advantage over 

 the others, and therefore to see why the variety which possesses 

 that character does not wholly supplant the others. Without 

 disputing the proposition that there are cases in which what 

 seems to us the " merest trifle," or really is so, may be of 

 selection value, the position that it is always so selected, 

 i. e. is always practically of selection value, appears untenable. 

 Such a position would be hardly consistent with the almost 

 universally persistent prevalence of variability as an attribute of 

 species which yet preserve general stability as such. And it 



* Observations showing, in any case, Tiow the different places in nature 

 are filled up, notwithstanding that in many cases there appears to be room 

 for a larger number of individuals of a species than permanently exists 

 would be valuable. 



PKOC. ENT. SOC. LOND., V. 1906, I 



