67 



seasons, whilst the more aberrant individuals are killed off, 

 and sooner or later eliminated. 



In spite of this general truth, it is well known that we do 

 occasionally find a marked aberration occurring among a 

 large number of specimens which differ but little from each 

 other. Such a form may bear no close resemblance to any of 

 the examples comprising the whole brood to which it belongs, 

 and yet may occasionally recur in other broods at more or 

 less distant intervals of time. Such individuals are known 

 as " sports," and are generally considered to be of little value 

 in the progress of organic evolution by those who believe in 

 the general efficiency of natural selection. Sports of this 

 nature are generally supposed to be atavisms ; and that this 

 view is possibly correct is fairly well proven by the well- 

 known Pyramcis cardui ab. dynii, odd examples of which 

 are occasionally captured at long-distant intervals in this 

 country and on the Continent, but which Fischer bred 

 rather freely by exposing the pupae to low temperatures, 3° 

 to 12° C, at a critical period of their development. In 

 Vanessa io, too, an occasional aberration is taken in nature 

 in which the ocellated spots are resolved into their con- 

 stituent parts, producing the series of pale submarginal 

 spots, which is a decided Vanessid character, in a compara- 

 tively unmodified form. In no part of the distribution of this 

 species is this known to be the normal form, yet Merrifield 

 was able to produce this form by icing the pupa at 33*^ F. for 

 twenty-two days, following this by placing it in a refrigerator 

 for twenty days, and then in a cool cellar for eighteen daj^s, the 

 imago, on emergence, showing the resolution of the ocellus 

 into a chain of small bright white spots, with a slight bluish 

 shade about them. There appears to be no doubt that the 

 result is produced by the effect of a low temperature on the 

 organic functions during histolysis and histogenesis ; in 

 other words, that it is an outward manifestation of an 

 inward physiological change. Such internal variation as is 

 here suggested appears to be due to the ability of the species 

 (or its progenitor) to respond in past times to a degree of 

 cold in its pupal stage now rarely experienced by it, the 

 species still having sufficient elasticity in this direction to 

 so respond if necessary, in which case the form produced is 

 an atavism, and probably approaches to some extent the 

 Vanessid stem from which V. io sprung. " Sports" in their 

 most marked forms are apparently outward manifestations 

 of the extreme possibilities of variations of physiological 

 functions which lie at the present time outside the normal 



