14 



THE EVOLUTION OF BIEDS, 



Mysterious 

 nature of 

 definite 

 variation. 



Climatic va- 

 riation ap- 

 parentlj' less 

 mysterious. 



Effects of 

 use and dis- 

 use, in re- 

 ality mys- 

 terious. 



Mysterious 

 nature of 

 other laws 

 of Nature. 



The force 

 which pro- 

 duces Evolu- 

 tion pro- 

 bahly analo- 

 gous to the 

 forces which 

 operate on 

 inorganic 

 matter. 



beneficial variation does cover all the ground. If Romanes be right in his estimate of the 

 swamping effects of interbreeding, it follows that all variations which survive must be 

 definite ; and the term definite variation, as opposed to fortuitous variation, must be used 

 in place of beneficial variation, in order to cover all the ground. 



The second objection to Teleological Variation, which applies equally to Definite 

 Variation, whether it be beneficial or otherwise, that it appears to be scarcely less 

 miraculous than special creation, is not very easy to dispose of. There are some 

 causes of variation which we can easily understand, as, for instance, the change from 

 yellow to red in the colour of a Canary, when it has been fed for some time on Cayenne 

 pepper. 



There are other causes which we readily accept without understanding them, as for 

 instance the effect of the climate of the north of Siberia, especially that of the peninsula of 

 Kamtschatka, in exaggerating the white in the plumage of many species of birds, such as 

 the Nuthatch, the Lesser Spotted Woodpecker, the Marsh Tit, the Magpie, &c. 



The effects of use and disuse, in lengthening the primaries of migratory birds, and 

 shortening those of residents, are so obvious that we cannot dissociate cause and effect, 

 though, if we were asked to explain the modus operandi, we should be obliged to conceal 

 our absolute ignorance of it, by the expression that long primaries are generally found 

 correlated with migratory habits. 



Are we using less scientific language when we say that white eggs are generally found 

 correlated with a nest in a hole, or some equally concealed situation, and coloured eggs 

 with an exposed position amongst objects somewhat similar to them in colour ? Although 

 we cannot explain either set of facts, it somehow seems more possible that the increased 

 use of the wings, in consequence of the adoption of migratory habits, should cause a 

 lengthening of the primaries, than that the greater risk of destruction, in consequence of 

 the adoption of an exposed breeding-site, should cause colour to be deposited on the eggs 

 in the oviduct. The connection between cause and effect in the latter case appears to be 

 so remote, so impossible to imagine, that we are tempted to call it miraculous. 



But are not the operations of other laws of Nature to all appearances equally 

 miraculous ? The egg of a Tortoise, and that of a Cormorant, are very similar in appearance. 

 The development of the embryo in each is at first very similar, but that of the one is 

 arrested at an earlier stage than that of the other, and had previously been proceeding on 

 lines slightly divergent from, though nearly parallel to, that of the other. We explain this 

 by assuming that each egg was produced by its respective parents with the self-contained 

 potentiality of developing, under certain physical conditions (of heat &c.), in a definite 

 direction. If the history of the embryo be an epitome of the history of the species for 

 almost countless ages, why may not the potentiality of future development be self-contained 

 within every individual, and only require the aid of its environment to modify within certain 

 limits its precise direction ? The necessity to develop, in a more or less definite and 

 beneficial direction, which appears to be characteristic of organic matter, may possibly be 



