480 THE POPULAR SCIENCE MONTHLY 
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 :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 Ginothera 
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 :? 
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 eall forth different responses, but, as is shown in the chapter on 
1“ Mutations, Variations and Relationships of the (inotheras,” Carnegie 
Institution of Washington, No. 81, p. 90, 1907. 
2 An Investigation of Evolution in Chrysomelid Beetles of the Genus 
Leptinotarsa,” Carnegie Institution of Washington, Publication No. 48, p. 295, 
1906. 
