622 



SCIENCE. 



[N. S. Vol. XXIV. No. 620. 



cause (2) of the swamping or extinguishing 

 of favorable variations by their intermating 

 with unfavorable or neutral conditions. 



Thus there is necessary some influence tend- 

 ing to cumulate variation independent of, or 

 antecedent to, selection. This influence or 

 causal factor in species-forming on a basis of 

 continuous variation must, therefore, be some- 

 thing whose result is determinate or ortho- 

 genetic variation. What can produce such 

 progressive variation doesn't for the moment 

 seem very obvious. But a prior consideration 

 for observation and experiment (pedigreed 

 breeding) may well be that of the determina- 

 tion of whether we can actually distinguish 

 the occurrence of the determinate variation. 

 If we can't discover its existence we need not 

 trouble our fevered wits with questions of 

 how it might be produced. 



I wish, therefore, to present some f acts^ con- 

 cerning a little green and black beetle that 

 infests our Californian flower gardens — one 

 Diabrotica soror or ' California flower beetle ' 

 by name — ^that seem to me to have intimate 

 relation to the problem just stated. 



This beetle has its black and green colors 

 arranged on its back (dorsal surfaces of the 

 wing-covers) in the form of twelve distinct 

 black blotches or spots on a green ground, six 

 spots in three transverse pairs (or two longi- 

 tudinal rows) on each wing-cover. At least 

 the original description of this species gives 

 this patterning, and systematic accounts and 

 revisions of the genus have always ascribed to 

 the species soror twelve separate black blotches 

 on a green (or yellow-green) ground. In 

 Horn's revision of the genus in 1893 (Trans. 

 Amer. Ent. Soc, V. 20, p. 89 ff.) the fact of a 

 tendency of the black spots to coalesce is 

 fleetingly referred to. But undoubtedly the 

 twelve-free-spots type is the pattern which is 

 accepted as the typical and usual one. 



In its larval stage this beetle lives as a 

 slender white grub underground, feeding on 

 the roots of alfalfa, chrysanthemum and vari- 

 ous other plants. It pupates in a small sub- 



^ See Kellogg and Bell, ' Studies of Variation 

 in Insects,' Proc. WasJi. Acad. Sci., Vol. 6, pp. 

 203-332, 1904, for a detailed account of the varia- 

 tion in Diabrotica soror. 



terranean cell near the surface and the adult 

 beetle, on issuance from the pupal cuticle, 

 makes its way aboveground and feeds on the 

 buds and opened flowers of roses, chrysanthe- 

 mums and almost any other of California's 

 favorite blossoms. The color pattern of the 

 adult is, of course (as the insect is one of 

 * complete metamorphosis ') definitive and 

 fixed as to both pattern and color at the time 

 of the first appearance of the adult above- 

 ground. 



By the aid of several industrious assistants 

 I have been able to collect from the same 

 locality in the same months each year a 

 thousand or more specimens of Diahrotica 

 soror in each of five separate years included 

 in the last decade. In addition, we have made 

 other collections from other localities in Cali- 

 fornia of series varying from a few score to a 

 thousand individuals. With the help of these 

 same indefatigable^ helpers all of these thou- 

 sands of beetles have been closely scrutinized 

 and honestly described with regard to the 

 actual condition of their elytral pattern. The 

 results of this work are graphically represented 

 in the accompanying ' frequency polygons ' 

 and statistical tabulations. 



From this scrutiny and compilation it is 

 apparent, (1) that in this patterning varia- 

 tions of the strictly fluctuating or continuous 

 sort exist, as was to be expected ; (2) that this 

 variation is strongly marked and hence readily 

 tabulatable, which is fortunate for our study; 

 (3) that this variation does not follow Que- 

 telet's law of error, in which characteristic it 

 departs from the usual condition of fluctuating 

 variation, but is not unique; (4) that this 

 variation , plainly sets strongly in a certain 

 specific direction, that is, tends strongly to- 

 ward the production of one particular new 

 type of pattern rather than toward dissipating 

 itself by futile equal attempts in various and 

 thus mutually extinguishing directions; and 

 (5) that this tendency is on the steady in- 

 crease in our own times, under our very eyes. 



The pattern variation is shown (by selecting 

 certain principal types appreciably distinct) 



^ Instructor R. G. Bell and students R. Patter- 

 son and B. E. Wiltz, 



