90 EVOLUTION IN COLOR-PATTERN OF THE LADY-BEETLES. 



David Starr Jordan has proposed a law of distribution of great value, 

 not because it is universal, for it is not, but, still better, because it is to 

 some degree indicative of the nature of evolution in the species consid- 

 ered. "Given any species in any region, the nearest related species is 

 not likely to be found in the same region nor in a remote region, but in 

 a neighboring district separated from the first by a barrier of some sort." 

 President Jordan has subsequently stated that "barrier " is here used in a 

 very broad sense. This law gives the state of affairs that we should 

 expect if evolution were mainly the work of isolation and were taking 

 place by even flow rather than by waves or mutations. It is doubtless 

 mainly his knowledge of the distribution of fish that led President Jordan 

 to this formulation. Its applicability to the vertebrates is quite wide. 

 Botanists do not seem to find it of as general applicability in plants; 

 indeed, in some genera, like Draba, the opposite condition prevails. In 

 invertebrates but few authors have considered it. It is important that 

 many groups should be examined with this in view, for it throws valuable 

 light on the method of evolution. We have seen that it is largely inappli- 

 cable for the major species because of their extensive overlapping. 



For the consideration of the distribution of varieties and minor species 

 I have drawn the distribution of the several derivatives of Hippodamia 

 convergens in fig. 91. The distribution of Hippodamia convergens itself is 

 not shown. It is found thoughout the area of the map. It will be seen 

 that Jordan's law fails here, for the closest allied forms occupy the same 

 region in all cases with the parent species and in many cases with an 

 allied variety. Thus the closely allied variety quinquesignata and variety 

 caseyi have ranges nearly coextensive in the west. So far as our data 

 goes, variety caseyi does not share the eastward extension of variety quin- 

 quesignata. It is characterized by the possession of three of the four dis- 

 tinctive characteristics of the variety quinquesignata and is so generally 

 associated with it that I believe future study in the eastern mountains 

 will reveal its presence. Hippodamia bowditchi dwells in a part of the 

 range of its nearest ally and is surrounded by it. The conclusion seems 

 inevitable that we are dealing here with systematic units that have not 

 arisen by an en masse evolution of all individuals in the locality, but have 

 arisen at first in a few individuals which by virtue of some degree of seg- 

 regation in heredity have escaped being swamped. This conclusion, 

 reached from the study of the distribution, is the same as that which the 

 study of the variation and heredity of these beetles leads to. 



Geographical isolation does not act in these beetles as the initial cause 

 of separation as it does with blending characteristics. It may assist the 

 variety in its progress, however, (1) by preventing too wide outcrossing 

 and thus causing the individuals of the variety to more frequently inter- 

 breed, and (2) by helping the variety to attain intersterility and thus a 

 specific status, in a way shown later. Specimens of Hippodamia spuria 

 isolated from its close ally, Hippodamia interrogans, are more alike in gen- 



