98 GENERAL REVIEW OP THE ORCHIDE.E. 



GEOGRAPHICAL DISTRIBUTION— CLIMATOLOGY. 



'I'lie geographical distribution of the most important genera, in a 

 horticultural sense, is fully sketched under each genus and further 

 illustrated by maps on which the habitats of the species, so far as 

 they have been ascertained, ai-e indicated. It is only necessary to 

 note here some general facts relating to the region over which the 

 epiphytal orchids are spread and the climatic phenomena of that region. 

 With the epiphytal orchids are associated some terrestrial and sub- 

 terrestrial genera whose habitats lie within this region, as the 

 Cypripedia (section Selenipeuia and sub-section Coriace.e), Phaius, 

 Thunia, Spathoglottis, Calanthe and others. Its geographical limits may 

 be broadly stated to be the 30th parallel of north and the 35th parallel 

 of south latitude; the region of epiphytal orchids is thence a broad 

 zone, having for its breadth a little more than one-third of the 

 entire distance between the poles and, roughly speaking, including 

 about three-sevenths of the land area of the globe. Beyond this 

 zone the epiphytal orchids have spread only into two localities 

 remote from each other, very sparingly indeed and as outlying 

 members of the genera to which they belong, northwards into 

 Japan and southwards into New Zealand. These two localities, 

 together with some of the islands of the Pacific Ocean on which 

 orchids included in tropical genera are found, are not shown on the 

 maps illustrating the climate of the region. 



But while the geographical limits of epiphytal and other orchids 

 belonging to tropical types are those defined above, a very important 

 modification has to be made with respect to the actual area over 

 which they are spread. While the temperature of the whole region 

 except on the summits of the higher mountain ranges is sufficiently 

 high to maintain epiphytal life, there are extensive tracts within it 

 where owing to physical causes the other equally essential condition, 

 that of humidity, is either altogether absent, or is present in 

 insuffi^cient quantity, or for too short a period to enable epiphytal 

 orchids to live. 



Thus in the eastern section there are — in Asia, the Arabian deserts, 

 the arid plains of Persia and north-west India, the table-land of the 

 Deccan and Mysore ; the greater part of the Australian continent ; and 



