574 Mr. C. F. M. Swynnerton on 



At the same time, neither this rule nor any other that T 

 have suggested can be expected to be universal, for other 

 considerations very often are by no means equal. A species 

 that has relatively recently begun to encounter either a more 

 rigid selection or a slackening of selection, is unlikely to 

 show the effects of the change so strongly as a species that 

 has been subjected to these conditions even in a somewhat 

 slighter degree for a far longer period. But degree should 

 be important too. Selection, again, immense as must be its 

 importance as the regulative factor in evolution, is by no 

 means the only thing to be considered. Environment may 

 frequently impose a variability on a species that the latter 

 will only be able to counteract, if at all, by the adoption 

 or accentuation of some defence perhaps quite other than 

 anything suggested above. In such a case as the Field- 

 fare's (always supposing it to be valid) the additional 

 *' fightability " may have been the result of the imposed 

 variability, not this of the other : though in few cases of 

 this kind is it really possible to say which is the cause and 

 which effect, or to do other than suppose that both developed 

 together, reacting on each other. 



Great caution is obviously necessary in the interpreta- 

 tion of particular cases. Thus my quotation (above) from 

 Mr. Wallis, which gives Fieldfare, Missel-Thrush, Blackbird, 

 and Song-Thrush as the order of " fightability " for these 

 four birds, is just spoilt as a suggestion of the effect of 

 graded fightability by the fact that the order of variability 

 transposes Missel-Thrush and Blackbird. Either, therefore, 

 relative fightability has nothing to do with relative variability 

 in the four species mentioned, or complicating factors must 

 also be taken into account — as no doubt they must in any 

 and every case. Again, one would have been tempted to 

 suggest the House-Sparrow's egg as an instance of varia- 

 bility resulting from relative inaccessibility, owing to the 

 bird's attachment to human dwellings, were it not for the 

 knowledge that wilderness-inhabiting members of the genus 

 (as P. arcuatus) also lay very variable eggs. Every case will 



