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In cerebrate animals the reaction of the individual is under the guidance of preceding impressions stored in the pallium and known as memories; whereas in the animals without a pallium all reaction is accomplished through stable mechanisms known as instincts. Both of these types of reaction are tropisms merely; but the former are labile, conditionable; whereas the latter cannot be modified.
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.
Loeb states, on the authority of Eigenmann, that all those forms which live in caves were adapted to life in the dark before they entered the cave, because they are all negatively heliotropic and positively stereotropic, and with these tropisms would be forced to enter a cave whenever they were put at the entrance.
The stone wall before the honest mechanist is the abolition of purpose, and design, an old insoluble problem upon his premises. Preach, until you are blue in the face, behaviorist tropisms, in which man is pushed and pulled about in his environment as are iron filings in a magnetic field.
Even those among the Amblyopsidae which live in the open have the tropisms of the cave dweller. But these latter are not blind, and the argument only tends to show that the blind fish Amblyopsis entered the caves before it was blind. Nocturnal animals generally must be said to be negatively heliotropic, but these usually have larger and more sensitive eyes than the diurnal.
Loeb all he did was to restate destiny, one of humanity's oldest ideas, in the terms of tropisms, infusoria and light. Omar Khayyam chemically reincarnated in the Rockefeller Institute. Nevertheless those who accept his theories have to admit that there is essentially no difference between their impulses and the rush of filings toward a magnet.
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