PLACE VARIATION. 93 



tion or modification of color pattern moves caudalward or distahvard, while 

 decrease moves cephalward or medianward. 



(c) On the wings the markings follow the same rules which were found 

 by Mayer (1902) to hold for the wings of Lepidoptera. 



(d) Large or extreme variations are determinate and always occur in direc- 

 tions corresponding to the maximum lines of fluctuating variations. 



(3) Range of variation. In all cases for any given character variation is 

 limited in both directions, but there seems to be no limit to the combinations 

 of characters in the formation of species. The species show differences in the 

 extent to which they exhibit variation in any character, in some the percent- 

 age of the total range of variation being high and in others low, and this is 

 a constant quantity, a characteristic of the species. 



(4) Correlation of variation.- All variations of color and structural char- 

 acters are strongly correlated. On the dorsal and ventral surfaces, and 

 between them, correlations in variation are equally strong, so that causes 

 which produce a variation in one part bring about either directly or indirectly 

 corresponding variations in other parts. 



PLACE VARIATION. 



The term "place variation" is used to designate a phenomenon more or less 

 general in plants and animals. It has long been recognized that animals and 

 plants vary from generation to generation, from season to season, from year 

 to year, and perhaps in longer periods. Fluctuations in environmental factors, 

 in the intensity of the struggle for existence, in fecundity, and other causes 

 bring about the condition that succeeding generations of the same animals are 

 not even in one restricted locality alike. As frequently as this place variation 

 must have been noted, it has been passed over as unimportant, and has not 

 been studied in animals and only slightly in flowering plants. The researches 

 of Burkhill (1895), MacLeod (1899), Ludwig (1901), Tower (1902), Shull 

 (1902), Yule (1902), Pearson (1903), Reinohl (1903), and Shull (1904) 

 have shown the existence of this phenomenon to an unrealized degree, and its 

 importance where studies of evolution and variation are in progress. As has 

 been pointed out by several authors, and especially emphasized by Shull 

 (1904), workers in the field of biometry are all too prone to ignore this 

 phenomenon. 



Shull states (p. 372) : 



The interpretations which students have based upon the assumption that seasonal 

 fluctuations do not occur will have to be greatly revised or discarded altogether, and 

 before we can appreciate the exact bearing of any case of variation upon the great 

 problems of evolution it will be necessary to know the laws governing that variation. 



