THE BIRDS OF A DROUGHT. 53 



Climatic conditions (temperature, rainfall and the like) 

 nutritive conditions, (nature and extent of food supplies, and 

 degrees of facility with which they can he obtained), these 

 latter very often mainly dependent on- the former, and com- 

 petitive conditions (in which I include the absence or presence, 

 not only of races consuming the same food, but also those 

 actively hostile), all combine with configural conditions to 

 determine range. 



These different classes of conditions operate with very vary- 

 ing degrees of potentiality where different classes of animated 

 life are concerned ; and even in the same class, in the case of 

 different families, and at times even genera. 



In tropical and sub-tropical climates, probably no one factor 

 exercises so powerful an influence over the distribution of land 

 birds (as opposed to shore and water birds) as the rainfall. 

 Our rainfall charts have not yet been worked out in sufficient 

 detail to enable me to present the matter in a complete shape, 

 but we have enough data to show to what a remarkable extent 

 the average annual rainfall influences the distribution of a 

 vast number of species. 



You fiud a species plentiful in a certain region, of which the 

 average annual rainfall is, say 100 inches and over ; leaving this 

 region the species is perhaps absolutely wanting for a thousand 

 miles, and then you re-enter an iso-ombral* tract and straightway 

 your species re-appears. 



Hitherto, while tracts have been classed according to average 

 temperature and half a dozen other averages, very little, if any, 

 attempt has been made, in this country, to class them according 

 to average rainfall, and yet in tropical and sub-tropical regions, 

 at any rate throughout this vast empire, nothing so distinctly 

 governs distribution. 



It is customary to talk of the Malayan fades, of the Fauna of 

 the Malabar Coast, the Assamboo Hills and part of Ceylon ; what 

 is this but that in these localities you recover the heavy rainfall 

 of the Malay Peninsular ? How the same species or represen- 

 tative forms found their way to these distant localities is another 

 question, but their survival in each is due primarily to the 

 extent of the rainfall. 



What gives such a plains of India fades to the dry upper 

 portions of Pegu, but the light average rainfall ? What 

 allows the Indo-Malayan species to run up westward along the 

 feet of the Himalayas, at any rate as far as the Ganges, but the 

 heavy rainfall ? 



* oju/5jos=raiu. ' 



