EXPERIMENTS IN HYBRIDIZATION AND TEMPERATURE. 285 



ment a certain line is always followed without much variation 

 between individuals. 



A specific line of development such as was obtained by cold 

 experiments was never evolved by the same species by warmth 

 experiments, nor was the obverse ever obtained. It was plainly 

 a case of direct influence. 



It was very different with frost and heat experiments. In this 

 case the whole of the material experimented with was never 

 influenced in one direction differing from the normal type. 

 Firstly, it must be remembered that it was the rule for the 

 greater part to remain unaltered in any way ; secondly, among 

 those that were influenced, even though, for certain individual 

 groups, a number of regularities were observable in the variations, 

 yet the one group as compared with another would often enough 

 vary in an entirely different direction, even in one and the same 

 experiment. Moreover, at minus temperatures, after a certain 

 limit had been reached even at great divergences— for example, 

 — 8° to — 18° C. — qualitatively the same variations were obtained, 

 although not in the same percentage ; this was also the case with 

 extreme degrees of heat. In the cold and warmth experiments, 

 on the other hand, a very few degrees were sufficient, if con- 

 stantly applied, to create vastly different forms. We even find 

 that the influence of heat on certain species of Vanessa will 

 solely produce forms which are certainly equivalent to certain 

 forms obtained by frost experiments, as the differences which up 

 to the present could be detected between aberrations obtained by 

 the influence of heat and certain aberrations obtained by the 

 influence of frost are overborne by the preponderance of simi- 

 larity in the structure of these two forms, and may perhaps be 

 entirely removed by further studies. 



From all these facts it follows that the second answer to the 

 question, " In what way do these heat and frost experiments 

 work?" is : 



They have no direct influence, but indirect, owing probably 

 to the fact that through the state of lethargy brought about by 

 them, conditions act which cause a variation in the future 

 insect in a particular direction ; and it is quite immaterial for the 

 setting up of this line of development whether the period of 

 lethargy is caused by frost, by heat, and perhaps also by any 

 other hindering influence. 



Now, what deductions do we draw from the aberrations ? 

 What can we think of them ? We have as yet given no explana- 

 tion of them, but only indicated that they are forms which, 

 although not confined to any locality or time of appearance, are 

 found in a state of nature, generally as great rarities, here and 

 there in the territory inhabited by the species. This says nothing 

 as to their real essence. 



