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dittercntiateil aloiiy; liiu's \vlu(;h hiivt'. n^sultcil in thcjii- picscut chiiiac- 

 teristics. On the one side I see variations which have become sntfi- 

 cie;it]y fixed to be consiih'red specific; yet w]»it;li can have no especial 

 beaiiu;j;" on the Ht'e necessities of the species, but are a consequence 

 rather of that universal tendency t) variation with wliich every student 

 of iiatnie becomes profoundly im]>ressed. Thus the wing inarkin<»s 

 vary from a darker general coloring, as in Prodi >.v us (Ctwseens^U* a more 

 uniform intermixture of the black scales among the white, as in cine- 

 rew,s', or a sparser intermixture thereof, as in pnlvrruhmfiis. The dispo- 

 sition of th.e black scales is in spots or bands, whether transverse or 

 longitudinal, as in marginatus, reticulatus, y-mversus, etc. These are 

 fortuitous variations, for I can not believe that the disposition of these 

 marks, where, as in these cases, they take every form that is conceiva- 

 ble, can be of any benefit to the species, any more than the mere varia- 

 tion in the number of lobes in the leaves of different oaks growing 

 under like conditions can be of any particular benefit to the species, 

 however useful to ns in classification. 



ON FORTUITOUS VARIATIONS. 



In my address before the Section of Biology of the American Asso- 

 ciation for the Advancement of Science, at Cleveland, in 1888, 1 have 

 discussed the various forms and causes of variation, and especially the 

 limitations of natural selection, stating expressly that this last -'deals 

 only with variations useful to the organism in its struggle for existence 

 and can exert no power in fixing the endless number of what, from 

 present knowledge, we are obliged to consider fortuitims characters," 

 and I have long recognized, from my studies of insect life, the existence 

 of these fortuitous variations. The subject has since been very well 

 elaborated by Prof. Ward in his communication to the Society (Decem- 

 ber 15, 1888) on "Fortuitous Variation as Illustrated by the Genus 

 Eui)atorium," and in his Annual Address (January 24, 1891) on " Neo- 

 Darwinism and Neo Lamarckism," and the Prodoxidje furnish an ex- 

 cellent illustration of this fortuitous variation. Yet at the same time 

 that we note this chance variation, as exemplified in a number of the 

 species of Prodoxus, which are mere ravagers or despoilers and have 

 not been brought into any special or mutual relations with the plant, 

 we have, on the other hand, in Proniiixt yi(ccaseUa, correlated with the 

 other striking structural modifications which have brought it into such 

 special relations with the plant, an elimination of all maculation or 

 markings upon the primaries, and a purely white coloring so fixed that 

 it shows absolutely no \ariation over half the continent. The structural 

 variation has been necessary, a consequence of effort, environment, and 

 natural selection. The color variation, on the contrary, has not been 

 absolutely necessary, yet has nevertheless gone on in lines wliich, tend- 

 ing to give gTeater protective resemblance to the flower, have in the 

 long run proved to be perhaps the L^ost advantageous. I thus recog- 

 nize three distinct lines of variation as exemplified in these Prodoxidse, 



