. Evolution and animal life; an elementary discussion of facts, processes, laws and theories relating to the life and evolution of animals . the author on the behavior ofthe lowest organisms. This work has shown that in these creatures thebehavior is not as a rule on the tropism plan—a set, forced method ofreacting to each particular agent—but takes place in a much moreflexible, less directly machinelike way, by the method of trial anderror. This method involves many of the fundamental qualities 430 EVOLUTION AND ANIMAL LIFE which we find in the behavior of higher animals, yet with the simples
. Evolution and animal life; an elementary discussion of facts, processes, laws and theories relating to the life and evolution of animals . the author on the behavior ofthe lowest organisms. This work has shown that in these creatures thebehavior is not as a rule on the tropism plan—a set, forced method ofreacting to each particular agent—but takes place in a much moreflexible, less directly machinelike way, by the method of trial anderror. This method involves many of the fundamental qualities 430 EVOLUTION AND ANIMAL LIFE which we find in the behavior of higher animals, yet with the simplestpossible basis in ways of action; a great portion of the behavior con-sisting often of but one or two definite movements, movements that arestereotyped in their relation to the environment. This method leadsupward, offering at every point opportunity for development, andshowing even in the unicellular organisms wrhat must be considered the beginnings of intelligence and of manyother qualities found in higher action doubtless occurs, but themain basis of behavior is in these or-ganisms the method of trial and FIG. 265.—Diagram showing howthe motile Protozoan, Stentor,reacts to light: A circular spacehalf in light and half in dark;the animalcules collect in darkarea; 1, 2, and 3 show thereaction of a specimen whichcame to the light line. (AfterJennings.) Different one-celled animals showdifferences in method or degree ofresponse to external protozoa will discard grainsof sand, crystals of acid, or otherindigestible objects. Such peculi-arities of different forms of lifeconstitute the basis of instinct. Instinct is automatic obedienceto the demands of external condi-tions. As these conditions varywith each kind of animal, so must the demand vary, and from this arises the great variety actu-ally seen in the instincts of different animals. As the de-mands of life become complex, so may the instincts become greater the stress of en
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