THE PALEONTOLOGIC RECORD 83 



Weismann made a great contribution to the progress of biology by 

 focusing attention on the germ cells, and although many of his specula- 

 tions 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 con- 

 clusions in two paragraphs i 1 



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 (Enothera 

 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 demon- 

 stration 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 de- 

 velopment by the conditions of moisture and temperature during the 

 germinal stages, and induced changes that were perpetuated in the off- 

 spring, the changed offspring at least in some instances mendelizing 

 with the parent species. He presents his conclusions in the following 

 words : 2 



A careful consideration of the various lines of experimentation recorded 

 and of the pedigree cultures and the data from observations in nature irre- 

 sistibly 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 tem- 

 porary 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 



^'Mutations, Variations and Relationships of the (Enotheras," Carnegie 

 Institution of Washington, No. 81, p. 90, 1907. 



a "An Investigation of Evolution in Chrysomelid Beetles of the Genus 

 Leptinotarsa," Carnegie Institution of Washington, Publication No. 48, p. 295, 

 1906. 



