THE RHOPALOCERA IN THE PYRENEES. 321 



Now what are the causes of these apparent incongruities ? 

 That nature refuses to be bound by the rules of geographers, 

 and conform with mathematical precision to the calculations of 

 physicists, without taking all the factors into consideration, is 

 plainly patent. The theoretical (climatological) zones defined 

 by geographers harmonise very well on the whole with the 

 actual (phyto-geographical) zones adopted by botanists through- 

 out the world, and Europe in particular ; but when our attention 

 is confined to a small area, they are often found to be incon- 

 sistent in detail. It is clearly evident that other factors should 

 be taken into account besides the mean annual temperature in 

 estimating the horizontal and vertical limits of the vegetation. 

 In many cases the minimum winter temperature or the maximum 

 summer temperature has to be provided for, and this I consider 

 furnishes the cue to the reason why the vine does not ascend to 

 such a height in the Hautes Pyrenees as it ought to do from the 

 point of view of the mean annual isotherm alone. Its non- 

 occurrence above the altitude of 1800 ft. is in my idea to be 

 accounted for by the temperature of the summer, beyond the 

 elevation at which the plant is successfully cultivated, being 

 insufficient to permit of the fruit ripening. A good deal of rain 

 falls in the region in question in the summer and autumn 

 months, which is probably the cause of the temperature being 

 insufiicient to bring the grape to perfection ; hence the reason 

 why it is not grown. 



In the case of many indigenous species of plants and shrubs 

 belonging to the warm temperate zone being unable to extend 

 the area of their distribution even to the lowest elevations in 

 this district, where the winter temperature is comparatively very 

 mild, the same influence possibly operates as with the vine, the 

 low temperature of the summer being likewise insufiicient to 

 ripen the seed, and thus not enable them to propagate their 

 species. Perhaps it is also partially to be accounted for by their 

 inability to compete with the more vigorous cold temperate forms 

 which, thriving superabundantly by reason of the profuseness of 

 the precipitation, do not afford a chance to the arid-loving forms 

 from the southern and north-eastern sides of the Pyrenees to 

 continue their kind. 



The fauna, and particularly the Ehopalocera, of the pseudo- 

 olive zone in the Hautes Pyrenees appear to coincide in their 

 distribution with the flora very well. The butterflies even at 

 the lowest elevations, with a few trifling and unimportant excep- 

 tions, partake wholly of the character of the north temperate 

 region. It is possible that the reason of this lies in large 

 measure in respect to the distribution of their pabula ; in other 

 instances species may be influenced directly by the amount of 

 the precipitation, and by the temperature, in the same way as 

 the plants. 



