310 Mr. E. B. Poulton. Essential Nature of the [Apr. 23, 



other kinds might produce some tendency in this direction. In 

 gardens there are many chances in favour of eggs being laid for 

 several generations on apple, and so with regard to the predominance 

 of any one species of Salix over a particular area. 



It would thus be possible for the two varieties to be rendered 

 locally distinct, for the localisation of a food-plant would overcome 

 both causes of variability — the liability to lay eggs on other plants 

 with different tendencies, and the chance of interbreeding between 

 the two varieties. If the two varieties could become stereotyped in 

 this way (and it has been shown that occasional recurrence to other 

 kinds of food on the whole produces no effect), there would result a 

 species with specific differences in the larva?, although the pupae and 

 imagos would remain identical. Instances very nearly of this kind 

 are, of course, well known. The above would certainly be true of 

 two isolated tracts in which food-plants of opposite tendencies existed 

 to the exclusion of the other kinds. If the tracts were adja- 

 cent it would still be true of nearly all the larvae in each of 

 them. 



It maybe argued that the food-plants are so intermixed throughout 

 this country that we should not expect to find the uniformity which 

 has hitherto seemed to prevail. But it must be remembered that 

 such uniformity is chiefly observed upon apple, and that the effects of 

 this food-plant are so strong that it needs a very powerful tendency 

 to overcome them. Furthermore, there is a certain amount of local 

 separation between apple trees in gardens and the various species of 

 sallow by the banks of streams and in damp lanes and hedgerows. 

 The species of Smerinthus are inactive and slow flyers in the perfect 

 state ; so there is every tendency towards interbreeding between those 

 in gardens, and towards the eggs being laid upon the food-plant which 

 is on the spot. These facts will account for the almost invariable 

 occurrence of the whitish variety upon apple. The case is very 

 different with the sallows. The various kinds occur in close prox- 

 imity, so that it is very common to find several so-called species of 

 Salix in a single osier-bed. The tendency of sallows as a whole is 

 rather towards the yellowish variety than towards the whitish, but 

 there is at all times the greatest facility for interbreeding, and for 

 laying eggs on many species of food-plant, even in the case of the 

 most sluggish insects. This explains, the irregularity of the larvae 

 when found on sallows, and the fact that the yellow variety is 

 perhaps more commonly found upon the trees with an opposite ten- 

 dency than the white variety upon trees tending towards the yellowish 

 form. I may add that the garden at Reading in which I found the 

 yellowish form upon apple, is on the outskirts of the town, and so 

 situated that the eggs might easily be laid by a female of which the 

 larva had fed upon sallow, for these trees are common at no great 



