98 THE ENTOMOLOGIST. 



changes that have been going on daring past ages, and are still 

 proceeding. The different zones of vegetation which one meets 

 with in ascending a mountain therefore present, in my mind, 

 their respective antiquity — by no means accurately so, as I have 

 already said ; but the higher one ascends the more ancient will 

 become the prevailing constituents of the flora of each particular 

 phyto-geographical belt, together with its accompanying fauna. 

 The higher zones constitute, as it were, the final sanctuaries of 

 many species which are unable to modify their characters in 

 harmony with the ever-changing organic conditions by which 

 they are surrounded, and are practically in the same position as 

 islands. As new conditions manifest themselves at the bases of 

 the mountain chains from time to time, in the appearance of 

 new types of animals and plants with more highly specialized 

 powers of adaptation, they will make their presence felt all 

 along the Hne from the bases upwards, the constituents of each 

 zone rebounding, as it were, on the one above, like waves upon 

 the sea-shore, the more ancient in turn succumbing to the next 

 in the order of succession. A typical instance of this change, 

 independent of any apparent alteration in the cHmatal conditions, 

 is to be found in the Scandinavian peninsula at the present day, 

 where the existing flora of the lower zones is gradually receding 

 before an Oriental one, which is slowly but surely making its 

 way westwards from Siberia. Some remarkable changes of a 

 somewhat similar character concerning the arboreal vegetation 

 have also occurred in Denmark within historic times. 



Such evolutions as these must be affecting the distribution of 

 the Ehopalocera to a considerable extent, for the larvas, being 

 dependent upon the plants for their sustenance, will either have 

 to retreat with their pabula, or succumb, unless they can con- 

 form themselves to their altered surroundings. Temporary 

 fluctuations or return waves will periodically manifest them- 

 selves, such, for instance, as the clearly ascertained fact of the 

 comparatively recent more elevated extension of the pine forests 

 in the Alps, and this will react upon the zones below, together 

 with their insect inhabitants, in the manner I have endeavoured 

 to exhibit. 



The organic environment therefore clearly plays a pre- 

 ponderating part in the distribution of organisms, while that of 

 the physical only a subordinate one, and this generally in an 

 indirect manner. While the former will operate almost exclu- 

 sively in curtailing their distribution downwards, the latter will 

 exert its influence principally in deciding their upper rather 

 than their lower limits. Another important item worthy of 

 special consideration and investigation by biologists is the fact 

 that the vertical and horizontal conditions affect the flora in a 

 difierent degree to that of the fauna. In the former kingdom 

 whole tribes and families seem to be similarly affected in respect 



