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ages to produce—great and sudden physical modifications might often 
produce the extinction of a race just as it was approaching perfection, 
and a hundred checks, of which we can know nothing, may have re 
tarded the progress towards perfect adaptation ; so that we can hardly 
wonder at there being so few cases in which a completely successful 
result has been attained, as shown by the abundance and wide 
diffusion of the creatures so protected.”—(Contributions. ftp. 68-69.) 
Whichever of these two authorities we follow, defensive 
colouring cannot be said to afford the most favourable 
illustration of the action of Natural Selection. If Mr. 
Bates’ view is adopted, the part played is very slight, and 
one does not see the necessity for the introduction of 
Natural Selection for which he contends. If Mr. Wallace’s 
view be correct, then this department does not offer a very 
favourable illustration of the action of Natural Selection, 
seeing that the process is so difficult and so protracted and 
so often accompanied by failure. 
But in this particular case, we are not at liberty to leave 
the actual amount of difference an open question and to 
argue from either hypothesis as to the possible result. For 
in the case of defensive colouring the initial change must 
be considerable before Natural Selection can come into 
action. There must be enough likeness to deceive before 
the individual so favoured can be saved. 
But, for the sake of argument, let us take the two 
hypotheses of slight changes and of considerable changes 
in each generation, and let us see what would be the effect 
upon the supposed action of Natural Selection. The 
agent in this case is the bird or beast of prey which acts 
efficiently by virtue of its keenness of vision, and it is one 
of the commonplaces of the theory that the vision of the 
enemy is quickened by the protracted process. Let us see 
what the result must be, first, in the case of considerable 
variations. If the process of Natural Selection begins at 
that point where certain insects pass into invisibility, or are