Page:Popular Science Monthly Volume 77.djvu/486

480 Weismann made a great contribution to the progress of biology by focusing attention on the germ cells, and although many of his speculations may be discarded, he was a great stimulator of thought. The work of MacDougal and Tower seems to show how the environment may act on the individual through the germ-cells and induce permanent changes in the progeny.

MacDougal has experimented with species of evening-primroses, by injecting salt solutions into the seed capsules, and summarizes his conclusions in two paragraphs:

The action of reagents having an osmotic and a chemical effect has resulted in the induction of mutants in the progeny of Raimannia odorata and Œnothera biennis. The mutants thus induced have been tested to the second and third generation and found to come true to their newly assumed characters.

The induction of mutants by the action of reagents is a conclusive demonstration of the fact that hereditary characters may be altered by external forces acting directly upon the reproductive mechanism. The action of the reagents used experimentally is simulated by many conditions occurring in nature.

Tower has conducted a series of experiments on species of beetles belonging to the genus Leptinotarsa. He endeavored to influence development by the conditions of moisture and temperature during the germinal stages, and induced changes that were perpetuated in the offspring, the changed offspring at least in some instances mendelizing with the parent species. He presents his conclusions in the following words:

A careful consideration of the various lines of experimentation recorded and of the pedigree cultures and the data from observations in nature irresistibly forces one to the conclusion that in these beetles the only variations of permanence are germinal, and that evolution is through germinal variations. Those germinal variations which arise in nature are permanent and the same variations, of the same degree of permanence, are produced in experiment. The diverse kinds of evidence produced in this and in preceding chapters all go to show that under varying conditions of their surroundings these beetles vary, and that as they become more and more extreme an increasing percentage of striking, permanent variations is found; and as I have just shown, it is possible in experiment to produce in this same way a variety of permanent modifications. From all this evidence, however, there nowhere appears the least trace of a suggestion of any specific action of the conditions of existence, but everywhere there appears only the action of environment as a stimulus, while the response is entirely determined by the organism. All of these variations of purely temporary and of permanent kinds resolve themselves into responses of the organism to the stimuli of its environment, but the nature of the response is entirely determined within the organisms. It is true that different intensities of the same stimuli call forth different responses, but, as is shown in the chapter on