1885.] Colouring of Phytophagous Larvce. 309 



According to this theory, the egg from which the former was hatched 

 had been laid by a different female from that which produced the 

 others. Such a conclusion is confirmed by the fact that the yellowish 

 larva was decidedly older than the others and became adult some 

 days earlier, the six intermediate larvae keeping together throughout. 

 I endeavoured to ascertain the past history of the parents of the 

 larvae with which I experimented, but Mr. Davis informed me that 

 his larvae had not been kept separate, and had been found on different 

 food-plants. 



It may be argued that there is probably great difference among 

 individuals of the same brood in the power of transmitting these 

 tendencies, and that this might explain some of the above-mentioned 

 irregularities. But the breeding experiments, as far as they go, 

 negative such a conclusion. There was very little difference between 

 the larvae of each separate lot, and considerable differences between 

 the lots (in some cases). The influences of the food-plant seemed to 

 be most rigidly regular in their effects on each division of the larvae : 

 in fact one might say that there were practically no individual dif- 

 ferences in the tendency towards a whitish variety, although the 

 tendency was very strong. 



There is one argument against the conclusion that these effects are 

 in any way due to the food-plants (but I do not see how it can stand 

 against the breeding experiments) : — the fact that the variably 

 developed system of brownish-red spots have only been found on the 

 yellowish-green larvae up to the present time. (Mr. Meldola's 

 instances point in this direction without exception, and so also with 

 my own.) It is, however, quite possible that these spots may be 

 ultimately found upon the other variety. If not, it would seem to 

 imply that the yellowish-green form, often brightly ornamented with 

 red spots, is more ancient than the other, and that reversion to it is 

 caused by certain food-plants. Against this comes the fact that the 

 red markings have all the excessive variability of a reversion character, 

 while the yellowish-green ground colour can be produced with great 

 regularity. Again, the spots occur upon larvae with all shades of 

 ground colour in 8. Populi. Looking at all the facts I cannot doubt 

 the cogency of the explanation offered above of the colouration being 

 due to the food- plant, and of the accumulation of the influence during 

 successive generations. It seems very probable that we have here an 

 instance of a larva of one species dividing into two. The tendency is 

 more of this kind than towards true dimorphism, for the two forms have 

 different food-plants. If a long series of generations upon one plant 

 produced a distaste for the other kinds, so that the larvae wandered or 

 died, it is then certain that natural selection would lead the females 

 to lay their eggs on the appropriate foods, and the separation would 

 be complete. Mere proximity of one kind of food and distance from 



