Just as iron filings always respond to the attraction of a magnet in the same way, just as certain combinations of mercury at the impact of a blow flare up with an explosion, just as ice when warmed melts and becomes water, and water when cooled to a definite point freezes into ice, so do the simplest living things seek out certain rays in the spectrum, certain temperatures, certain chemical conditions and avoid others. Not only unicellular organisms do this, but also comparatively highly developed animals, such as the daphniæ, for if light is sent through a prism into a vessel containing water, these little creatures collect at the violet end of the spectrum; such as the wood-lice, which hate the light and creep into dark crevices; such as gnats, which are attracted by the sun and dance in their hundreds in its rays. Moreover, we meet with a similar phenomenon in man. We, too, in winter and spring seek the sun and in summer the shade; in the cold season the warm stove attracts us; bad smells put us to flight, sweet scents of flowers allure us. The simplest automatic reflex actions are at the root of these attractions and repulsions, exactly the same as with the daphniæ, wood-lice and gnats. Only we are able to control and suppress these reflex actions which the lower animals apparently cannot.
Anthropomorphic modes of thought easily mislead us into thinking that the processes we observe in lower animals are due to an exercise of will power. We draw near to the fire in winter because it is pleasant, but we can quit it if duty calls us into the cold streets. One is apt to imagine that the simple organisms also experience pleasant and unpleasant feelings, that they try and avoid the latter, that the daphnia seeks the violet rays because it likes them, that the wood-louse flees the light because it dislikes it; in fact, that these creatures possess a consciousness which becomes aware of and distinguishes between pleasing and displeasing impressions, and that they possess a will which responds to these impressions with suitable reactions. Very distinguished scientists have been unable to resist the temptation to assume in the lower animals, even in unicellular organisms, the existence of processes with which we are familiar in the human consciousness. William Roux introduces us to a "psychology of protista," and W. Kleinsorge goes so far as to maintain the existence of "cellular ethics," and to devote himself to research into its laws. The work of both these biologists is as fascinating as the most beautiful fairy-tale, but it is probably the creation of a lively and fertile imagination, just as the fairy story is.