Man breaks into the circuit of the natural inorganic forces and arrests them and controls them, and makes them do his work—­turn his wheels, drive his engines, run his errands, etc.; but he cannot do this in the same sense with the organic forces; he cannot put a spell upon the pine tree and cause it to build him a house or a nursery.  Only the insects can do a thing like that; only certain insects can break into the circuit of vegetable life and divert its forces to serve their special ends.  One kind of an insect stings a bud or a leaf of the oak, and the tree forthwith grows a solid nutlike protuberance the size of a chestnut, in which the larvae of the insect live and feed and mature.  Another insect stings the same leaf and produces the common oak-apple—­a smooth, round, green, shell-like body filled with a network of radiating filaments, with the egg and then the grub of the insect at the centre.  Still another kind of insect stings the oak bud and deposits its eggs there, and the oak proceeds to grow a large white ball made up of a kind of succulent vegetable wool with red spots evenly distributed over its surface, as if it were some kind of spotted fruit or flower.  In June, it is about the size of a small apple.  Cut it in half and you find scores of small shell-like growths radiating from the bud-stem, like the seeds of the dandelion, each with a kind of vegetable pappus rising from it, and together making up the ball as the pappus of the dandelion seeds makes up the seed-globe of this plant.  It is one of the most singular vegetable products, or vegetable perversions, that I know of.  A sham fruit filled with sham seeds; each seed-like growth contains a grub, which later in the season pupates and eats its way out, a winged insect.  How foreign to anything we know as mechanical or chemical it all is!—­the surprising and incalculable tricks of life!

Another kind of insect stings the oak leaf and there develops a pale, smooth, solid, semi-transparent sphere, the size of a robin’s egg, dense and succulent like the flesh of an apple, with the larvae of the insect subsisting in its interior.  Each of these widely different forms is evoked from the oak leaf by the magic of an insect’s ovipositor.  Chemically, the constituents of all of them are undoubtedly the same.

It is one of the most curious and suggestive things in living nature.  It shows how plastic and versatile life is, and how utterly unmechanical.  Life plays so many and such various tunes upon the same instruments; or rather, the living organism is like many instruments in one; the tones of all instruments slumber in it to be awakened when the right performer appears.  At least four different insects get four different tunes, so to speak, out of the oak leaf.

Certain insects avail themselves of the animal organism also and go through their cycle of development and metamorphosis within its tissues or organs in a similar manner.