Davidson — Biological Studies of Aphis rumieis L. 321 



mechanism associated with the chromosome complex, it seems feasible to 

 expect that changes in the environment may, by influencing the metabolism 

 of certain individuals, indirectly affect the natural sequence of chromosome 

 segregation, and so affect sex-production. 



This would allow of two conflicting views — internal mechanism and 

 external factors — being brought into line, and would explain the result shown 

 in figure 3. 



External factors, therefore, may be regarded as exerting an influence by 

 retarding, restricting, or favouring the appearance of sexual forms, and not 

 as determining them. 



III. — SUMMAEY. 



The early aphidologists considered that food and temperature conditions 

 were the important factors influencing the apterous and winged forms in 

 Aphids. This view appears to be upheld by the later experiments of Ewing 

 and Shinji. 



From cytological investigations on Aphids and recent breeding 

 experiments, it appears highly probable that the sequence of winged and 

 apterous forms is largely due to some internal, inherent tendency. W. v. ? ? 

 tend to produce a. v. ? ? , and a. v. ? 2 to produce either a. v. ? ? or a 

 mixed progeny, including a very variable percentage of winged forms. The 

 apterous condition is to be regarded as an adaptation to seasonal food and 

 temperature conditions. The great variability in the numbers of winged 

 forms produced by apterous individuals is an important point to consider. 



Similarly, food and temperature conditions were considered the important 

 factors affecting the development of sexual forms. Later cytological 

 investigations show that the appearance of the sexual forms is associated 

 with changes in the chromosome complex. The agamic generations appear 

 lo be interpolated between the winter egg and the sexual generation as an 

 adaptation to seasonal conditions. The approach of winter conditions would 

 normally be the factor aft'ecting the bringing to an end of the partheno- 

 genetic phase and the appearance of the sexual forms. 



In some cases the production of sexual forms may be superseded by 

 continued parthenogenetic reproduction, certain agamic forms either 

 reproducing slowly throughout a mild winter, or lying dormant and con- 

 tinuing reproduction the following season. In isolated cases, certain agamic 

 individuals may be affected physiologically by some factor or factors acting 

 on their metabolism, so that they do not respond to the inherent stimulus to 

 develop into sexual forms. These individuals normally, in winter, would die, 

 but under favourable conditions may continue agamic reproduction. 



2i42 



