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- 



