VARIATION 321 
scientific analysis have been lacking. Either the same strain as was 
subjected to artificial conditions was not grown for comparison under 
natural conditions or else the conditions themselves were not suffi- 
ciently well controlled to permit of certain analysis. It is interesting 
to note that the pomace fly, Drosophila ampelophila, which has pro- 
duced more mutations so far as we know than any other organism, 
was subjected to the effects of ether on a grand scale and under 
controlled conditions by. Morgan, but that not a single mutation was 
observed to result from this treatment. However, mutations have 
subsequently appeared again and again in cultures of “wild”’ flies not 
only of this species but also of other species of Drosophila. Thus it 
appears that germinal variations frequently occur independently of 
external stimuli. It also seems that a tendency to produce mutations 
may be inherited. 
With animals the best known experiments on the artificial pro- 
duction of germinal variations are those of Tower who worked with 
the Colorado potato beetle, Leptinotarsa decemlineata, and related 
species. Like other arthropods these beetles are more directly under 
the influence of temperature changes at least than are warm-blooded 
animals. ‘Tower first determined the period in ontogeny when ex- 
ternal stimuli will affect the germ cells. He found that in Leptino- 
tarsa the germ cells do not become susceptible to external stimuli 
until after the time in ontogeny when the color pattern of the individ- 
uals subjected to the stimuli can be influenced. He found that eggs 
were most susceptible just before and during maturation and this 
observation is in agreement with those of Fischer, Standfuss, Weis- 
mann and others who have conducted similar investigations. Tower 
concluded that certain individuals from the germ cells of a stimulated 
parent “‘show intense heritable variations, whereas those not acted 
upon do not show these changes.”’ Most of the inherited variations 
involve changes in the pigmentation of the body parts. In certain 
cases there was an actual change in the color pattern. It is to these 
results that Tower attaches the greatest significance inasmuch as 
most similar experiments have not succeeded in causing pattern 
changes. In spite of the elaborateness of Tower’s methods consider- 
able skepticism exists regarding the validity of his conclusions, and 
this has not been lessened by the non-appearance of confirmatory 
data. In a recent paper he reports the production of very striking 
germinal modifications in L. decemlineata as a result of subjecting a 
morphologically homogeneous race to an extreme change in environ- 
