6 EINAR LÖNNBERG, BIRDS. 



or there may be some clumps of papyrus. In places where the ground is kept only mode- 

 rately moist this moisture procreates a rich vegetation of herbs and grass which is much 

 liked by certain birds. There is no lake in the country through which this expedition 

 went. 



Every biologist knows that the zoogeographical boundary-lines cannot be drawn up 

 as definitely as the political. This holds true especially for the higher animals and such 

 which with great ease can nio ve actively, or with great f acility can be transported passively 

 and then acclimatize themselves in a new country. For many animals there are, however, 

 certain physical obstacles which directly put a limit to their further distribution. In 

 other cases the climate is a hindrance to distribution. With regard to flying and warm- 

 blooded animals like birds it is usually neither one nor the other of these two factors which 

 directly and immediately decides whether a certain species shall be able to extend its 

 range or not. B ut it is a combination of many different physical, meteorological, clima- 

 tological, and above all biological factors which together form the conditions of life for 

 an animal. With aid of its wings a bird can surmount a mountain-ridge, pass över stret- 

 ches of water, and deserts etc. If such an animal has plenty of food and drink, it is of 

 less importance if the temperature is some degrees more or less. It can nevertheless 

 subsist. But there are many other things that are needed for the welfare and the 

 propagation of the species as for instance suitable breeding-places, suitable food for the 

 young, possibility of avoiding too powerful enemies, and many other things. Only if all 

 these conditions are fulfilled, a species can establish itself on a new ground, or maintain 

 itself on the old. Some species are more delicate and touchy and suffer from the least 

 alteration, or the lack of any of the necessaries of life, or they may be so strictly adapted 

 to a certain mode of life that they cannot at all change their habits. Such species are 

 more dependent, and must consequently have a more restricted habitat. This does not 

 necessarily mean, however, that a highly specialised form must have a small range be- 

 cause the conditions of life to which it is adapted may extend över a great area in which 

 the natural conditions may differ in different parts in many other respects which for that 

 particular species are of less importance. 



Other species again are strong enough to support themselves, and propagate more 

 independently of most exteriör factors, and such species are therefore able of a very wide 

 distribution. 



If the conditions of life were uniform över a whole continent such animals, which 

 like birds have means and power of active distribution, would soon become evenly distri- 

 buted all över that' continent, independently of centres of distribution. There is, however, 

 no such continent. In consequence of topographical and climatological factors certain 

 areas are more or less uniform and offer certain similar combinations of conditions of life, 

 which differ from those of other geographical areas. A result of this is that within such 

 an area the fauna becomes uniform in the same degree as the uniformity of the same condi- 

 tions upon which it is dependent. In such a way zoogeographical provinces or districts 

 are formed. But within such a large area as that spöken of above, which broadly speak- 

 ing is to some degree uniform, there are other minor areas which are still more strictly 

 uniform within certain limits. For the same kind of reason as the large area constitutes 



