PRESIDENTIAL ADDRESS. 623 



Tn the more northern forms of the fire-butterfly, Chrysophanus (Polyommatus) 

 phlceas, the upper surfaces of the wings are of a bright red-gold or copper colour 

 with a narrow black margin, but in southern Europe the black tends to extend 

 over the whole surface of the wing and may nearly obliterate the red-gold colour. 

 By exposing pupa? of caterpillars collected at Naples to a temperature of 10° C. 

 Weismann obtained butterflies more golden than the Neapolitan, but blacker 

 than the ordinary German race, and conversely, by exposing pupa? of the German 

 variety to a temperature of about 38° C., butterflies were obtained blacker than 

 the German, but not so black as the Neapolitan variety. Similar deviations from 

 the normal standard have been obtained by like means in various species of 

 Vanessa by Standfuss and Merrifield. Stand fuss, working with the small tortoise- 

 shell butterfly (Vanessa urticat), produced colour aberrations by subjecting the 

 pupae to cold, and found that some specimens reared under normal conditions 

 from the eggs produced by the aberrant forms exhibited the same aberrations, 

 but in a lesser degree. Weismann obtained similar results with the same .species. 

 E. Fischer obtained parallel results with Arrtia raja, a brightly coloured diurnal 

 moth of the family Bombycidae. Pupa? of this moth were exposed to a tempera- 

 ture of 8° C, and some of the butterflies that emerged were very dark-coloured 

 aberrant forms. A pair of these dark aberrants were mated, and the female pro- 

 duced eggs, and from these larva? and pupa? were reared at a normal temperature. 

 The progeny was for the most part normal, but some few individuals exhibited the 

 dark colour of the parents, though in a less degree. The simple conclusions to be 

 drawn from the results of these experiments is that a proportion of the germ- 

 cells of the animals experimented upon were affected by the abnormal tempera- 

 tures, and that the reaction of the germ-cells was of the same kind as the reaction 

 of the somatic cells and produced similar results. As everybody knows, Weis- 

 mann, while admitting that the germ-cells were affected, would not admit the 

 simple explanation, but gave another complicated and, in my opinion, wholly 

 unsupported explanation of the phenomena. 



In any case this series of experiments was on too small a scale, and the separate 

 experiments were not sufficiently carefully planned to exclude the possibility 

 of error. But no objection of this kind can be urged against the careful and 

 prolonged studies of Tower on the evolution of chrysomelid beetles of the genus 

 Leptinotarsa. Leptinotarsa — better known, perhaps, by the name Doryphora — is 

 the potato-beetle, which has spread from a centre in North Mexico southwards 

 into the isthmus of Panama and northwards over a great part of the United 

 States. It is divisible into a large number of species, some of which are dominant 

 and widely ranging ; others are restricted to very small localities. The specific 

 characters relied upon are chiefly referable to the colouration and colour patterns 

 of the epicranium, pronotum, elytra, and underside of the abdominal segments. 

 In some species the specific markings are very constant, in others, particularly in 

 the common and wide-ranging L. decemlineata, they vary to an extreme degree. 

 As the potato-beetle is easily reared and maintained in captivity, and produces 

 two broods every year, it is a particularly favourable subject for experimental 

 investigation. Tower's experiments have extended over a period of eleven years, 

 andjie has made a thorough study of the geographical distribution, dispersal, 

 habits, and natural history of the genus. The whole work appears to have been 

 carried out with the most scrupulous regard 'o scientific accuracy, and the author 

 is unusually cautious in drawing conclusions and chary of offering hypothetical 

 explanations of his results. I have been greatly impressed by the large scale on 

 which the experiments have been conducted, by the methods used, by the care 

 taken to verify every result obtained, and by trie great theoretical importance of 

 Tower's conclusions. T can do no more now than allude to some of the most 

 remarkable of them. 



After showing that there are good grounds for believing that colour production 

 in insects^ is dependent on the action of a group of closely related enzymes, of 

 which chitase, the agent which produces hardening of chitin, is the most im- 

 portant. Tower demonstrates by a series of well-planned experiments that colours 

 are directly modified by the action of external agencies — viz., temperature, 

 humidity, food, altitude, and light. Food chiefly affects the subhypodermal 

 colours of the larvae, and does not enter much into account; the most important 

 agents affecting the adult colouration being temperature and humidity. A slight 

 increase or a. sliaht decrease of temperature or humidity was found to stimulate 



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