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differentiated along lines which have resulted in their present charac- 
teristics. On the one side I see variations which have become suffi- 
ciently fixed to be considered specific; yet which can have no especial 
bearing on the life necessities of the species, but are a consequence 
rather of that universal tendency to variation with which every student 
of nature becomes profoundly impressed. Thus the wing markings 
vary from a darker general coloring, as in Prodoxus wenescens, to a more 
uniform intermixture of the black scales among the white, as in cine- 
reus, Or a Sparser intermixture thereof, as in pulverulentus. The dispo- 
sition of the black scales is in spots or bands, whether transverse or 
longitudinal, as in marginatus, reticulatus, y-inversus, ete. 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 us 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, I 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 fortuitous 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 [llustrated by the Genus 
Eupatorium,” and in his Annual Address (January 24, 1891) on ‘“ Neo- 
Darwinism and Neo-Lamarckism,” and the Prodoxide 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 Pronuba yuccasella, 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 variation 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 which, tend- 
ing to give greater protective resemblance to the flower, have in the 
long run proved to be perhaps the most advantageous. I thus recog- 
_ nize three distinct lines of variation as exemplified in these Prodoxide, 
