CH. X.] NATURAL SELECTION. 31 



digression. To the general validity of that theory, or even 

 to the validity of the more special hypothesis coucernins; the 

 uses of concealment or of conspicuousness, the success o. the 

 foregoing explanation is not essential, — since its possible 

 inadequacy may very well be due to the incompleteness of 

 our grasp upon all the details of this particular case. But, 

 returning from this digression to our main thesis, and con- 

 sidering the general significance of the phenomena of colour, 

 we see that, in addition to those most general phenomena of 

 organic life which demand for their explanation the Dar- 

 winian theory, there is at least one special class of pheno- 

 mena which that theory is competent to explain even in 

 minute details. And there are other special classes of 

 phenomena to which it has been applied with equally re- 

 markable success. But when a theory, deduced from the 

 observed general facts of organic life, and invoking no 

 agencies but such as are known to be in operation, is found 

 on trial to account for such an enormous mass of special 

 facts, for wliich no other valid explanation has been pro- 

 pounded, — we may well say of it, as Laplace said of his own 

 Nebular Hypothesis, that the chances in favour of its being 

 a true explanation are many thousand million to one. 



