GENERAL PRINCIPLES OF DISTRIBUTION 



nearest land. From the direction of the wind during the four days previous 

 to the occurrence, it appeared to be beyond doubt that the insects had come 

 from the islands named, and must therefore have crossed nearly a thousand 

 miles of ocean ! Large numbers of insects lay their eggs in timber, and 

 hence the agency of floating logs in aiding dispersal must be taken into 

 account, especially as such creatures in the egg-state may retain their 

 vitality for a considerable time. 



Marine animals live under more uniform conditions than those on the 

 land, but their means of dispersal may be briefly touched upon. Mammals, 

 such as Seals and Walruses, sometimes undertake long sea journeys, -while 

 the wanderings of the huge Cetaceans seem only to be limited by considera- 

 tions of temperature. The same remark applies largely to fishes, but with 

 them the depth of water may also have its influence on their dispersal. 

 Fresh-water fishes may travel from one large river-system to another when 

 the sources of the streams are near enough to each other, and through 

 changes in land-level the direction of streams may be altered, and lead to 

 considerable corresponding changes in the fauna. Lastly, the eggs of fishes 

 may be, and no doubt often are, carried from one district to another by 

 aquatic birds. 



CONDITIONS LIMITING DISPERSAL. 



Against the various phenomena favouring dispersal which have been 

 noticed above, must be reckoned certain factors which act in the way of 



species to a varying degree of heat and cold, and on the whole it may be 

 said that temperature does indeed play a not unimportant part in determining 

 or limiting the range of animals. It may be added, that the closest depend- 

 ence in this respect upon temperature is to be found ainong marine organisms. 

 Indeed, this is often the only condition acting as a limit. The temperature 

 of the water undoubtedly checks the distribution of sea-fishes, and in the 

 case of shallow-water species a great depth acts as a perfect barrier. 



It has been mentioned above, that in the case of non-flying terrestrial 

 animals, such as Mammals, it may be taken that an arm of sea exceeding 

 20 miles in width acts most effectively as a barrier to dispersal. The 

 few cases in which animals swim across even this distance can only be 

 regarded as possible methods of dispersal and not by any means the rule. 

 When, therefore, we find a great similarity in the faunas of two regions 

 separated by a wide tract of water, we must conclude that there has been in 

 past ages a land connection between them. As will be seen later on, truly 

 oceanic islands, i.e., islands not having been connected with a continental 

 area within comparatively recent geological times, are practically without 

 any Mammals, Reptiles or Amphibians, while on the other hand they possess 

 both Birds and Insects. A wide river may also act in the same way as an 

 arm of the sea, though to a less extent. Thus, in the plains of the Amazon, 

 the river separates entirely distinct sets of species of Monkeys, Birds and 

 even Insects. In the case of species inhabiting high altitudes, valleys may 

 act in precisely the same way. 



SKETCH-MAP OF THE TRUNK LINES FOLLOWED BY BIRDS IN THEIR MIGRATIONS. 



After Prof. J. A. Palmen. 



limiting distribution. In many cases the range of a species is coincident 

 with the area possessing the precise features suitable for its welfare. Thus, 

 for example, strictly arboreal Mammals, such as Apes, Lemurs and Sloths, can 

 only exist within the limits of the great forests to which they are adapted. 

 Again, mountain-loving species do not thrive in the lowlands, and the result 

 of this is that the colonies of such animals are sometimes widely separated 

 and even altogether isolated. The Chamois, for example, is found in the 

 Alps, the Pyrenees, and the Caucasus, but is entirely absent from the inter- 

 vening districts, while the Mammals inhabiting the elevated regions of 

 Tibet are singularly distinct and peculiar. 



Another important factor which has an influence on geographical 

 distribution is that of temperature. The opinions of authorities vary greatly 

 as to the amount of limitation due to the effects of temperature alone. 

 Examples can readily be found which prove that temperature very consider- 

 ably affects distribution, while on the other hand others are as easily met 

 with which apparently support the opinion that this factor is not of very 

 great importance. The fauna of the Arctic Regions is of such a character 

 that some zoogeographers would constitute a separate circumpolar region 

 which is inhabited by animals markedly distinct from those of more 

 southerly areas. Again, the South American Mammals, known as Vicunas 

 and Guanacos, have a distribution which decidedly proves that they are 

 influenced by temperature. In Peru and Ecuador they inhabit the high 

 regions of the Cordillera, while in the more temperate country of the 

 Argentine Republic and Patagonia they roam over the low-lying plains. 

 These animals, therefore, being addicted to a cool climate, cannot exist 

 elsewhere except at a considerable elevation. The Dotterel, too, in Great 

 Britain nests on our highest mountains, and on the fringe of the Arctic 

 Ocean at sea-level. On the other hand, an animal may be spread over an 

 area presenting a considerable range in temperature. The Tiger, as we have 

 already seen, is found not only in the hottest jungle districts of India, but 

 also at considerable heights in the Caucasus and the Altai Mountains,' and 

 even in the cold Manchurian plains. Examples like the latter may of 

 course be interpreted to prove the great adaptability of certain isolated 



That high mountain ranges act effectively as barriers to dispersal is 

 shown by the fact that the species inhabiting one slope of such a range are 

 often entirely distinct from those on the other; moreover, the fishes living 

 in the rivers rising on different sides of the same range are likewise of 

 different species. The influence of mountains is further borne out by the 

 curious difference we find between the Old and New Worlds in this respect. 

 In North and South America the chief ranges run from North to South, 

 and consequently the animals are only limited in their range by conditions 

 of temperature. In Europe and Asia, on the other hand, the principal 

 ranges extend in a direction more or less parallel to the Equator, and hence 

 we find in these continents a larger proportion of species with restricted 

 distribution. 



Lastly, deserts play a not unimportant part in limiting the range of 

 certain animals, especially those possessing weak powers of locomotion. 

 Thus, animals like the Amphibia, which require moisture for the younger 

 stages of development and which cannot wander much, find even a small 

 tract of arid ground an insurmountable obstacle. The Sahara has for a 

 very long period separated Northern Africa from the rest of the continent, 

 and so distinct are these areas in regard to their faunas that they are by 

 general agreement allocated to different zoological regions. 



Many other influences not yet mentioned may control the distribution of 

 animals in various ways. Some groups are dependent for their welfare, or 

 even their very existence, upon the presence or absence of other groups, and 

 this is notably the case with carnivorous animals and their victims. Again, 

 parasitic animals and their hosts have necessarily the same distribution, 

 either wholly or in part, while insectivorous birds can of course only exist 

 where there is an abundance of the particular group or groups upon which 

 they feed. Vegetation is also an important factor in determining the 

 limits of certain species. A butterfly, for example, would soon become 

 extinct were the food-plant of its Caterpillar to fail, and the same 

 applies to frugivorous birds and other groups indulging in a vegetarian 

 diet. Thus the whole fauna and flora of a district is bound together by 

 a complicated network of particular conditions, and the slightest altera- 





