. Elementary biology; an introduction to the science of life. Biology. TROPISMS AND THE BEGINNINGS OF SENSE 225 that the Paramecium has an idea of nice or nasty. But it is very plain that the protozoa are repulsed by the presence of sand grains and attracted by the presence of various kinds of bacteria. They will swallow the bacteria and pass the sand grains by. There is no doubt, however, that the difference between their reaction toward food and their reaction toward inert matter or toward injurious matter is due to a certain relation between the chemical constitution of the protoplasm and t
. Elementary biology; an introduction to the science of life. Biology. TROPISMS AND THE BEGINNINGS OF SENSE 225 that the Paramecium has an idea of nice or nasty. But it is very plain that the protozoa are repulsed by the presence of sand grains and attracted by the presence of various kinds of bacteria. They will swallow the bacteria and pass the sand grains by. There is no doubt, however, that the difference between their reaction toward food and their reaction toward inert matter or toward injurious matter is due to a certain relation between the chemical constitution of the protoplasm and the chemi- cal constitution of the outside substances. We should hardly be any more justified in saying that the ameba likes meat juice than we should be in saying that water dislikes oil. In one case, as in the other, the reactions de- pend upon certain relations be- tween the chemical compositions of the two reacting substances. Water does not choose to dissolve sugar and to leave sand undis- solved ; neither can we be sure that a protozoan chooses its food, notwithstanding the fact that it does take some kinds and reject other kinds of objects or materials. It is only when we come to the higher animals that we may begin to speak of choice in this sense ; and even among the highest animals most of the selecting and rejecting depends entirely upon reflexes and instincts rather than upon thought or feeling; that is, they depend upon the structure of the organism and upon the composition of certain organs rather than upon a conscious purpose or discriminating Fig. 88. General reaction In many one-celled animals every stimu- lation brings about the same response. In the Paramecium the animal, when it runs into an obstacle, whether physical or chemical (O), immediately reverses its movements, backing off a little way, turning to one side, as shown by the arrows, and starting off along a new path. Please note that these images are extracted from scanned page images that may
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