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It is chemotropism, not solicitude for its offspring, which drives the flesh fly to lay its eggs on decaying meat.

Several kinds of tropisms are recognized, such as phototropism or heliotropism, reaction to light; thermotropism, reaction to heat; electrotropism or galvanotropism, to electric current; geotropism, to gravity; chemotropism, to a chemical; rheotropism, to current; thigmotropism or stereotropism, to contact; and chromotropism, to color.

The latter fact, as well as the extraordinary fastidiousness, so to speak, of parasites in their choice of hosts or of organs for attack, point to reactions on the part of the host-plant, as well as capacities on that of the parasite, which may be partly explained in the light of what we now know regarding enzymes and chemotropism.

Here, too, must be classed also all the innumerable phenomena of Heliotropism, Geotropism, Rheotropism, Chemotropism, and other tropisms, in which the sun, or the earth, or currents, or chemical stimuli so affect a form of life—plant, alga, or spore—that it disposes its own movements or the arrangements of its parts accordingly, turning towards, or away from, or in an oblique direction to the source of stimulus, or otherwise behaving in some definite manner which could not have been deduced or predicted from the direct effects of the stimulating factors.

These are “reductions” of the higher phenomena of life to the terms of a lower and simpler process of “stimulus,” that is to say, to chemotropism in the second case and something analogous in the first.