376 EXPERIMENTAL STUDY OF INHERITANCE 



" From the simple conditions discovered by Mendel there has 

 arisen through the work of the last decade an array of observa- 

 tions tending to show that the Mendelian phenomenon is not 

 in many instances as distinct and simple as one might wish, and 

 at present diverse kinds of variability in the behaviour of char- 

 acters are described and attributed, in some instances, to several 

 different kinds of latency, to gametic coupling, to variable 

 potency, to variable dominance, and so on. The situation 

 essentially is this, that as investigation has progressed it has 

 been discovered that not one, but a host of determining factors 

 (I use the word factor as meaning something that makes possible 

 a given result, with no idea expressed or implied as to the nature 

 of this factor) are operative in the production of alternative 

 inheritance ; and in the attempt to preserve the letter of the 

 law of Mendelian theory of unit-characters with segregation in 

 gametogenesis, a host of hypotheses have been developed in 

 order to save the original theory " (W. L. Tower, igio). 



Tower's Experiments. — Great interest attaches to the ex- 

 periments of Prof. W. L. Tower (1910) on crossing species of 

 potato-beetle (Leptinotarsa). In these experiments the chief 

 variables were the conditions surrounding and incident upon 

 the germ-cells at the time of fertilisation, and it was found that 

 changes in the external conditions (temperature, humidity, etc.) 

 are associated with changes in the alternative (Mendelian) 

 inheritance. He succeeded " in creating a series of behaviours 

 in which the same characters are dominant to the complete 

 exclusion of others ; dominant to a lesser degree, or in which 

 there is a complete blend between the two in the F 1 generation, 

 or the appearance of both parental types in F 1 and both breed 

 true." 



The question of dominance, according to these experiments, 

 is not entirely dependent on the constitution of the germ-cells, 

 it is partly dependent on the external conditions operative on 

 the germ-cells at the time. In short, conditions external 





