to feed as usual.  One of them lit on the edge of this tray, and was just going to hop in, when she spied the caterpillar.  Immediately she began bobbing her head up and down, but was afraid to go nearer.  Another joined her, and then another, until at last there was a little company of ten or twelve birds, all looking on in astonishment, but not one ventured into the tray; while one bird, which lit in it unsuspectingly, beat a hasty retreat in evident alarm, as soon as she perceived the caterpillar.  After watching for some time, Weissmann removed it, when the birds soon attacked the seeds.  Other caterpillars also are probably protected by their curious resemblance to spotted snakes.

[Illustration:  FIG. 10.—­THE CATERPILLAR OF THE SMALL ELEPHANT HAWK-MOTH (Chaerocampa porcellus).]

Moreover, we may learn another very interesting lesson from these caterpillars.  They leave the egg, as we have seen, a plain green, like so many other caterpillars, and gradually acquire a succession of markings, the utility of which I have just attempted to explain.  The young larva, in fact, represents an old form, and the species, in the lapse of ages, has gone through the stage which each individual now passes through in a few weeks.  Thus, the caterpillar of Chaerocampa porcellus, a species very nearly allied to the Elephant Hawk-moth, passes through almost exactly the same stages as that species.  But it leaves the egg with a subdorsal line, which the caterpillar of the Elephant Hawk-moth does not acquire until after its first moult.  No one can doubt, however, that there was a time when the new-born caterpillars of the small Elephant Hawk-moth were plain green, like those of the large one.  Again, if we compare the mature caterpillars of this group of Hawk-moths, we shall find there are some forms which never develop eye-spots, but which, even when full grown, correspond to the second stage of the Elephant Hawk-moth.  Here, then, we seem to have species still in the stage which the Elephant Hawk-moth must have passed through long ago.

The genus Deilephila, of which we have three species—­the Euphorbia Hawk-moth, the Galium Hawk-moth, and the Rayed Hawk-moth—­is also very instructive.  The caterpillar of the Euphorbia Hawk-moth begins life of a clear green color, without a trace of the subsequent markings.  After the first moult, however, it has a number of black patches, a white line, and a series of white dots, and has, therefore, at one bound, acquired characters which in the Elephant Hawk-moth, as we have seen, were only very gradually assumed.  In the third stage, the line has disappeared, leaving the white spots.  In the fourth, the caterpillars have become very variable, but are generally much darker than before, and have a number of white dots under the spots.  In the fifth stage, there is a second row of white spots under the first.  The caterpillars not being good to eat, there is, as has been already pointed out,