212 COIvORATlON IN LEPTINOTARSA. 



With these several species the responses in color changes were always defi- 

 nite and in the same directions as those already described for decemlineata. 

 Likewise, they were not the effects of specific agents, but of stimuli which 

 accelerated or retarded the processes of pigmentation. All responses reached 

 the maximum at once, showed no accumulated effects of repeated stimulation, 

 disappeared as soon as the stimuli were removed, and did not reappear until 

 they were applied again. 



GENE^RAI, RESUI.TS DEPRIVED ^ROM ^XPE^RIMl^NTAI, MODIFICATION OF COLOR. 



In all investigations dealing with variation we must, whenever possible, 

 distinguish clearly between variations which arise in the germ plasm and 

 those which arise in the soma after fertilization, or during ontogeny. It is 

 therefore important in experimental work that the conditions of experiment 

 be applied at such a time that either the germ cells or the soma alone may be 

 affected, this being the only method by which we shall be able to arrive at a 

 solution of the vexed question of the part taken by somatic variations in 

 evolution. 



In my experiments with Leptinotarsa I have been fortunate in that this 

 much-desired result has been easily accomplished on account of the fact that 

 the germ cells do not begin growth until after the time in ontogeny when the 

 color patterns can be influenced by external stimuli. Although the germ 

 cells are present during all the time of experiment, they are in the division 

 periods, in which condition, as far as I can discover, they do not seem to be 

 influenced by stimuli. It is during the growth and maturation periods that 

 they are most sensitive, and these in Leptinotarsa do not begin until after 

 coloration is completed. It was possible, therefore, during ontogeny, and 

 especially during the development of the adult coloration, to apply strong 

 stimuli to which the germ cells were not at that time sensitive, and obtain a 

 transient somatic variation, and later on to apply the same stimuli during the 

 period of the growth of the germ cells and obtain modifications which were 

 similar, but which were permanent in heredity and in subsequent generations. 

 Although later on I shall devote an entire chapter to a consideration of this 

 subject, a statement of the above results seems necessary at this point. 



The correctness of my methods of experimentation and the observations 

 upon which it is based is proven by the fact that, among the thousands of 

 somatic variations which I have produced in my experiments on color modifi- 

 cations, not one has ever given the least indication of permanency in succeed- 

 ing generations, or in crossing; nor can they he preserved by the most intense 

 selection after the removal of the stimulus which produced them. This possi- 

 bility in Leptinotarsa of so surely isolating somatic and germinal variations 

 and their production has enabled me to conduct experiments and obtain 

 results which would not be possible with the material used by other workers 

 along this line. 



