72 



SCIENCE 



[N. S. Vol. XXVI. Ko. 655 



Concerning this crossing in nature and its eflfect 

 upon juncta I shall have more to say in a later 

 paper. The full explanation of the extinction of 

 juncta is to be found in the fact that the two 

 species cross freely in nature, and that this nat- 

 ural crossing has resulted in a most interesting 

 and peculiar case of prepotency in one species and 

 of submergence in the other. 



Chapter II., on Variation in Lepiinotarsa, 

 gives us a detailed account of the variation, 

 not merely of the adults, but also of the early 

 stages. Schemes are invented for tabulating 

 the different kinds of variation, and abundant 

 statistics are offered. There is a most inter- 

 esting section on " Place Variation," a term 

 used to signify "the variation in any given 

 species in the same locality from generation 

 to generation, or from season to season, or 

 year to year." Professor Tower says: 



As far as I am able to determine from observa- 

 tions and experiment, place variations result in 

 no permanent modifications, nor do the changes 

 seem to be inherited. . . . This place variation 

 must necessarily be a troublesome factor in the 

 study by biometric methods of evolution, geograph- 

 ical variation or selection. In my own work it 

 has been the rock upon which many cherished 

 schemes have been wrecked, and I suspect it has 

 not yet completed its destructive work. If one 

 would study any of the broader problems of evolu- 

 tion by biometric methods he must iirst of all 

 determine whether in the material chosen for 

 study this phenomenon exists, and if it is found, 

 too great care or too long a time can not be spent 

 in the elimination of this factor. At present I 

 know of but one method of doing this — that is, by 

 collecting data and material over a sufficiently 

 long period to determine the range of this form 

 of variability. Unhappily this demands time, 

 patience and often funds which the investigator 

 will not or can not afford. The failure to take 

 into cojisideration this place variation vitiates the 

 validity of a large part of the biometric work that 

 has been done, and there is no reason to think 

 that it will be otherwise in the future. 



It appears from the tables given, that there 

 was usually an oscillation, occupying about 

 four generations, from one extreme to the 

 other. This oscillation even occurred in a 

 laboratory experiment, as shown in A, Plate 

 26. Any one observing the changes through 



four generations only might consider that he 

 had a beautiful case of progressive evolution, 

 when, in the fifth, he would be again at the 

 starting point! It is certainly conceivable, 

 and if true of extraordinary interest, that 

 this phenomenon is not wholly due to external 

 causes, but is in part the outcome of a rhythmic 

 pulsation of life, as it were, analogous to that 

 which produces the well-known phenomenon 

 of alternation of generations. 



Professor Tower sums up his views on place 

 variation under four heads, the fourth de- 

 serving to be quoted in italics. 



Ill place variation, ichenever there occurs an 

 extreme oscillation of the population there is an 

 accompanying production of an unusually large 

 percentage of extreme variations or mutants. 



This he observed in nature, and also under 

 experimental conditions. The general fact 

 has been known or suspected for many years — 

 see, for instance, the variable Virginia colony 

 of Helix nemoralis, and other like eases — but 

 Professor Tower has planted what was some- 

 what of a " castle in the air," firmly upon the 

 ground. 



In the laboratory experiments it was found 

 that mere selection of the extremes of normal 

 variation (say light or dark) did not affect 

 the type, the polygons made from the offspring 

 of light or dark beetles being practically co- 

 incident. When, however, strong environ- 

 mental stimuli (say dryness and moisture) 

 were applied, it was found that marked " place 

 variation " occurred, resulting in distinct poly- 

 gons for the lots so treated, the two not even 

 closely approaching, much less overlapping. 

 By the application of suitable conditions, to- 

 gether with rigid selection, it was easily pos- 

 sible to produce two very distinct races: not, 

 however, much exceeding the extremes of 

 normal variation for the species. Neverthe- 

 less, when these things were abated, the beetles 

 soon went back to the normal condition of the 

 species, no trace of the past excentricity re- 

 maining. 



I have said that no trace of the excentricity 

 remained, but while this was exactly true for 

 the bulk of the material, while the excentricity 



