Pheasants and Hungarians 125 



"shotgun" pattern. It would seem unlikely that they would extend in a broad 

 and almost continuous belt across four States. 



Food and cover in the gross, or quantitative sense, would seem to be more 

 abundant and just as varied in the southern zone where exotics have failed, as in 

 the northern zone where they have succeeded. Here, too, a patchy rather than a 

 zonal pattern would be expected, if they were the primary cause of success or 

 failure. As already stated, food and cover seem to have a great deal to do with 

 the abundance of both species in the area of success, and their resistance to shoot- 

 ing, but we are here wrestling not with questions of how well they succeed, 

 but why they succeed at all in one zone and not in another. 



Accordingly we seem to be forced to fall back on invisible factors as the 

 place to look for causes of success or failure of exotic plantings. 



Kinds of Invisible Factors: Pathology and Nutrition. It is con- 

 ceivable that the zone of failure represents the range of some disease organism 

 or parasite, or of some intermediate host which transmits a disease or parasite to 

 these birds. The phenomenon of straggling (this is, early thrift followed by 

 later failure) may be explained by supposing that when the plant is first made, 

 the disease organism is scarce and hence ineffective, but that it increases with the 

 planted birds. When it finally increases faster than the birds, it either kills 

 them, or by destroying their fecundity or disturbing their sex ratio, it causes 

 them to succumb to other enemies. Such reactions are well known and commonly 

 follow transplantation in both plants and animals. 



This may be called, for short, the pathological hypothesis. It would be 

 the most probable explanation of the behavior of exotics, but for one funda- 

 mental and seemingly conclusive contradiction: the thrifty behavior of pheasants 

 on game farms within the zone of failure. Captive birds would seem to be 

 accessible to the same carriers or intermediate hosts as wild ones, with the possible 

 exception of brush-inhabiting non-mobile insects, or mammals excluded by wire 

 pens. Unless such carriers or hosts can be found, the pathological hypothesis is 

 apparently untenable. 



One other possible invisible factor has occurred to the author. This postu- 

 lates some nutritional deficiency, or qualitative defect of the available food, in 

 the zone of failure. It has been expressed in two forms, which will now be 

 described. 



Glaciation Hypothesis. While making the game survey of Iowa it was 

 observed that successful plants of both pheasants and Hungarians were all con- 

 fined within the exterior boundary of the "Wisconsin" glacier, that is, that they 

 were confined to soils of recent glacial origin. The hypothesis was advanced 

 that some plant growing on these soils, or some substance, such as kind of lime or 

 gravel, contained in them, was necessary to the welfare and breeding vigor of 

 exotics of this region. 



The evidence subsequently gathered in the other States seemed at first to 

 support this hypothesis. All successful establishments seemed to occur on glacial 



