1864.] 405 



obsolete, but on what plant they occurred I do not know. From these 

 facts I infer that H. alternata^ when it inhabits the elder, has the elytral 

 vittae distinct and black, and that, when it inhabits the willow, there 

 is a strong tendency for those vittae to become obsolete, less strong 

 perhnps when it feeds on Salix longifolia, and more strong when it 

 feeds on S. humilis. Finally, I know from ray own boyish experience, 

 that when the common silk worm is fed entirely upon lettuce leaves 

 from the egg to its adult stage, it always spins not yellow, but whitish 

 silk ; but whether this variation in the color of its secretions is corre- 

 lated with any variation in the larva or imago state of the insect, I 

 cannot say. Many other such examples will occur to every intelligent 

 and observing field-entomologist. Varieties of the above character, i. e. 

 where certain unimportant characters in the insect are correlated with 

 the food-plant, while at the same time there is no sufficient reason to 

 doubt that the two varieties freely intercross, I propose, for convenience' 

 sake, to call Phytophagic Varieties. We may observe that Phytopha- 

 gic Varieties, like Dimorphous and Trimorphous forms, (/'/•oc. Ent. Soc. 

 Phil. pp. 221 — 3) sometimes — at all events if the dwarfed form of f A/-. 

 scalaris be considered merely as a variety — offer an exception to the 

 general law, that the absence of intermediate forms proves diversity of 

 species. 



Even with the little we know of the Laws of Inheritance, we mi"ht 

 infer a priori, that when from peculiar circumstances a Phytophagic 

 Variety, including both the sexes, has fed for a great many generations 

 upon one particular plant of the number inhabited by the species to 

 which it belongs, it would be likely to transmit to its descendants in 

 the imago state a tendency to select that particular plant upon which 

 to deposit its eggs. We know, for example, that young pointer pup- 

 pies, when taken into the field, will frequently point game without any 

 instruction or training whatever, though the habit of pointing is clearly 

 an acquired and not a natural habit, and must have been transmitted 

 to them from their ancestors in virtue of the Laws of Inheritance. If, 

 then, it should so happen, that, owing to the presence of but a single 

 species of the plants ordinarily fed upon by a particular species of in- 

 sects, or to other causes, eggs have been uniformly deposited by a Phy- 

 tophagic Variety upon the same plant for an indefinitely long series of 

 generations — say fifty, or a hundred, or a thousand, or ten thousand — 



