CHARLES DAEWrN". 133 



insects, more seeds, or more vigorous seeds, will bo perfected in 

 that variety, and it vrill tlius have a better chance of perpetuating 

 itself than will the original form. 



The struggle for existence amongst animals is too obvious to 

 require illustration. Their increase is chiefly checked by one 

 species preying upon another, by disease, and by insufficiency of 

 food. As with plants will the healthiest best withstand the 

 attacks of insects, so with animals which prey upon one another 

 will the strongest or most wary gain the victory. In both, the 

 effect of the struggle must be to perpetuate and increase beneficial 

 modifications. In animals which have no means of defending 

 themselves against attack, some subtle device is necessary, and 

 that is generally some mode of concealment. The most efficient 

 way to escape notice or attack is to resemble something else 

 which is not subject to attack. This is the origin of mimicry, 

 which takes several forms. Many insects escape destruction by 

 resembling the flowers, leaves, twigs, or bark of the trees on 

 which they feed. Certain beautiful and conspicuous butterflies 

 have a disagreeable odour which renders them obnoxious to 

 birds ; others, belonging to a different genus, and having no 

 offensive odour, resemble these in their habits and colour, and 

 so escape destruction. Natural selection offers the only con- 

 ceivable explanation of both these forms of mimicry, the tendency 

 to mimicry being increased by the most mimetic individuals 

 having the best chance of surviving and bearing offspring which 

 inherit their peculiarities. 



These are merely a few illustrations of the application of the 

 principle of natural selection to the explanation of phenomena 

 which without it are utterly inexplicable. But, after all, this 

 principle merely supplies a missing link in a chain of causation 

 still discontinuous, unless we accept an inherent tendency to vary 

 as an efficient cause of variation. It enables us to understand 

 how, when a beneficial variation takes place, that variation is 

 perpetuated, but it does not show why beneficial variations occur. 

 Natural selection is merely a term for the survival of the fittest 

 by the destruction of the unfit. It cannot produce anything, 

 but it is a necessary factor in evolution, for without it the less 

 fit would be as likely to endure as the fittest, and there would 

 be no progress. It embraces the theory of Lamfirck, for by natui-al 

 selection only can the modification of organs by use and disuse 

 owing to changes in environment, be preserved and accumulated 

 in the right direction for progress ; and it accounts for living 

 things fitting the conditions of their existence without being 



VOL. VII. — PART IV. 10 



