The “jumping bean” of Mexico is a segment of the triply divided fruit of a large spurge, which is called _Sebastiana palmeri_. The spurges are known in England as little green-leaved annuals, with yellow-green flowers and a milky juice. Botanists call them the _Euphorbiaceæ_, and in that “natural order” are included the boxwood tree and some tropical trees of great value and importance. None other than the Brazilian indiarubber tree, Hevea, of which we hear so much nowadays, its rubber to the value of £14,000,000 being exported every year from Brazil, is one of them. So also is the Chinese candle-tree, which furnishes a tallow-like fat, made into candles in China. Others are the croton oil and the castor oil shrubs, natives of India, and the manihot or tapioca plant. The fruits of _Sebastiana_ (the jumping bean) are very much like those of the croton; and as there are crotons (though not the one of the purgative oil) in abundance in Mexico, it has taken some time to make sure that the “jumping bean” is not the fruit of a croton, but that of the allied plant _Sebastiana_. It appears that there is no commercial value for this plant, and that those capsules which happen to contain a grub and move are collected from the ground by the native Mexican boys and sold as curiosities. The moth (_Carpocapsa saltitans_) lays its eggs on the Sebastian shrub, and the young grub, on hatching, eats its way into the young fruit when the latter is still quite soft and the seed unformed, and so leaves no hole to mark its entrance. As the fruit swells the grub eats out the seed and surrounding pulp of the segment of the fruit into which it entered early in life. By the time the fruits are dry and fall to the ground the caterpillar is fully grown. Of course, it is only a very few of the capsules which are thus invaded by a grub. The question very naturally arises, “Why should the caterpillar put itself to the great muscular effort of making the little capsule in which it is contained jump and move over the ground?” It seems probable that these movements are made in order to bring the capsule from an exposed position when it falls on to the ground—where it might be crushed or eaten by some animal—into a position of shelter, either into a hole, or under some stone or fallen wood. The warmth of the sun in an exposed position excites the caterpillar to activity, which ceases when it has reached the shade offered by some protecting cranny. In the same way I have applied artificial heat and, alternatively, shelter from heat, so as to cause the movements or the resting of the jumping bean in a London sitting-room.