130 PEARCE : VARIETIES OF BANDED SNAILS. 



But, now, inasmuch as the sheep would find the shells 

 eaten, even when ground down small by the action of 

 their molars, but an unpleasant addition to their ordinary 

 food, the shells if only seen in time would be avoided and 

 rejected. Hence the result that only the inconspicuous or 

 mottled forms would be consumed with the grass ; the more 

 easily detected variety ornala, with its black banding and its 

 white shell, would escape, because perceived by the sheep as it 

 fed, and so avoided. I'hus the white and banded shell, or the 

 shell with a tendency towards this banded and white form, is, 

 under the circumstances of the sheep pasture, an advantage to 

 these varieties of Helix caperata. And the van ornata on the 

 sheep pasture must tend to increase, and the inconspicuous 

 mottled form, which is liable to be eaten by the sheep, to 

 decrease. 



For we have learnt from the author of the 'Origin of 

 Species' that any variation (like this, towards dark banding on 

 a whitish shell) however slight and from whatever cause pro- 

 ceeding, if it be in any degree profitable to the individuals of a 

 species in their relations to other organic beings and to their 

 physical conditions of life, will tend to the preservation of such 

 individuals, and will be generally inherited by the offspring. 

 ('Origin of Species,' chap, ii, p. 49). 



So has been the history, we may perhaps believe, of the 

 variety ornata. That slightest tendency in some of the mottled 

 forms to make bands gave those particular individuals an advan- 

 tage over the run of the ordinary mottled forms in the struggle 

 to live on the same pastures where sheep customarily come to 

 feed. This variation towards the variety ornata, which from its 

 conspicuousness was safe from the sheep's mouth, tended to 

 increase in numbers, so that after a lapse of time we can under- 

 stand how it is we now find on the well-used pastures of the 

 downs the variety ornata so plentiful ; but on the sand-hills and 

 such like spots which sheep do not frequent it is entirely absent 

 or only occurs very scantily. 



LC, vi., Oct,, 1889. 



