Facts and Implications of Orthogenesis 403 



of horns among the deer and elks, and so on. One is im- 

 pressed with the tendency for these special forms or organs 

 to push on and on to greater size or complexity. 



In a species manifesting gene mutations there some- 

 times appear several mutations of the same gene with the 

 result that individuals or lines show a series of graded char- 

 acters. In the case of the fruit fly there is a short series of 

 gradations between the fully formed compound eye and the 

 complete absence of eye, related apparently to a change in 

 a certain gene (Fig. 99). Such a series strongly suggests 

 the operation of some orthogenetic influence. As we have 

 already seen, however (page 229), there are cases of such 

 series in which the successive members are known to have 

 originated independently. That is to say, we may have a 

 graded series without necessarily assuming an orthogenetic 

 tendency. 



Facts and Implications of Orthogenesis 



For several years before and after the turn of this cen- 

 tury, students under Professor Vernon Kellogg at the Leland 

 Stanford, Jr., University, recorded the wing pattern of the 

 beetle Diabrotica soror found on the campus (Fig. 100). 

 During ten years there was an obvious shift in the prevailing 

 color pattern, as shown by the statistical record (Fig. loi). 

 There was nothing in the circumstances to indicate a selective 

 process at work. There was no apparent advantage to the in- 

 sects in one pattern as against the other. Dr. Kellogg inter- 

 preted the results as an indication of orthogenetic variation, 

 unrelated to the struggle for existence or to any discoverable 

 external conditions. Other cases have been reported both 

 among plants and among animals under direct observation; 

 and Luther Burbank, who had exceptional opportunities to 

 note variations in large numbers, was convinced that varia- 

 tion is determinate, although he found enough indetermi- 

 nate variations among the plants which he cultivated to yield 

 a multiplicity of new forms for every species. 



