GENERAL PRINCIPLES OE DISTRIBUTION 



tion in any detail may upset the balance of the whole and lead to far- 

 reaching and unforeseen results. Interference by man himself has entirely 

 changed the faunal aspect of certain countries. Examples may be found in 

 the disappearance of animals like the American Bison, Quagga, Dodo, Great 

 Auk and many others, which have succumbed to the sporting or destructive 

 propensities of the human species or the so-called march of civilisation, 

 while the opposite result has been reached in the case of certain species 

 introduced purposely or by accident into regions far removed from their 

 native home. Such a case is that of the Rabbit in Australia, which, intro- 

 duced a comparatively short time ago, has increased and spread with 

 amazing rapidity. 



OTHER CONDITIONS AFFECTING DISTRIBUTION. 



There is strong evidence in support of the theory that all the great 

 continental areas and all the chief ocean basins have been for a vast period 

 in the same general position. At the same time the presence of sedimentary 

 rocks over all the great land masses proves that practically every portion 

 has been at one time or another under water. This general permanence of 

 situation, coupled with a constant change of form, must have exercised a 

 paramount influence on the distribution of animal life. Attention has been 

 drawn by more than one authority to the fact that at the present day all 

 the greatest masses of land appear to radiate from the Arctic Regions. 

 Behring Strait is the only break, and this is not only shallow, but inter- 

 rupted by islands. Hence it is possible for a traveller to start at Cape 

 Horn and finish at the Cape of Good Hope without ever being out of sight 

 of land. 



Let us now consider the effect of past changes. As one portion of a 

 continent gradually became submerged its inhabitants would be forced to 

 migrate to a fresh area, there to experience a new set of conditions, including 

 competition with other groups of animals, which they might succeed in 

 exterminating or to which, on the other hand, they themselves might 

 succumb. Again, the isolation of a portion of land by the submergence of 

 the surrounding areas might lead to the development of special forms of life, 

 and these might, by the subsequent raising of a submerged area, be enabled 

 to reach another portion and mingle with its fauna. Thus the ever-changing 

 relationships and proportions of land and water must have resulted in a 

 great complexity of conditions, sufficient, indeed, to account for many of the 

 peculiarities of distribution known to us. 



The slow and gradual changes of climate which certain regions have from 

 time to time undergone must also be reckoned as a most important factor in 

 regulating the distribution, evolution and extinction of the various forms of 

 life. There is abundant geological evidence that the Arctic and Ant- 

 arctic Regions were favoured during the Secondary and Tertiary Periods 

 with a much warmer climate than they possess at present, while, on the 

 other hand, the remarkable cold spell in the northern temperate regions, 

 which is generally spoken of as the "glacial epoch," must also be held 

 responsible for most important changes in the character of the fauna of these 

 regions. For example, in Pliocene times Tigers, Camels, Elephants, Rhino- 

 ceroses and Hyamas inhabited the British Islands, as well as other parts of 

 Europe. But as the glacial epoch gradually crept on, most of the temperate- 

 loving forms of life would be compelled to retreat further and further south, 

 while some would become either extinct altogether or modified to suit the 

 changed conditions. The presence of certain Arctic forms, such as the Musk 

 Ox, in Britain, and their eventual disappearance, may thus be accounted 

 for. 



The evolution of new species and genera, and the extinction of old forms, 

 implied by the foregoing considerations, may be effected in something like 

 the following manner. The natural tendency of any species which is suc- 

 cessful in the ever-waging struggle for supremacy is to gradually spread over 

 a wider and wider area. The increase in the number of individuals which 

 is implied in such a case indeed necessitates an extension of the space occupied, 

 otherwise the supply of food would be insufficient. As a species extends its 



range and comes to occupy ground more and more remote from its original 

 home, the outlying members may be confronted with a new set of conditions, 

 climatic, physical, or otherwise. Such a contingency will, by a process of 

 natural selection, lead to a modification of the original species, and this 

 process being repeated as the successful animals spread more and more, a 



SKETCH-MAP OF THE TRUNK LINES FOLLOWED BY BRITISH 

 BIRDS IN THEIR MIGRATIONS. 



After W. Eagle Clarke. 



group of closely allied species will in time be evolved, which may be regarded 

 as a distinct genus. Hence, in considering the geographical distribution of 

 existing genera, which are nothing but groups of allied forms, due regard 

 must be given to their origin and development by natural selection. 



On the contrary, if several species, possessing similar habits, occupy the 

 same area, the competition becomes much more intense,' and the weaker or 

 less fitted may in course of time be exterminated. In such a case, an 

 extension of range or the removal by some means, casual or otherwise, of a 

 portion of the individuals to a remote or isolated district, may save the 

 species from extinction. It may continue to exist in its new home without 

 the slightest modification, while in the more crowded original area new 

 varieties are being evolved. 



The principles involved in the two preceding cases have probably been 

 in force for an immense period of time, and hence the present anomalies of 

 distribution may be partly explained by an examination of the remains of 

 extinct forms. Fossil species may be discovered in regions which lie between 

 areas which are now isolated and inhabited by their descendants. The work 

 of the palaeontologist must, therefore, be of immense aid to the student of 

 zoogeography, and, as Wallace justly remarks, " even with our present scant}^ 

 information, we are able to throw much light upon the past history of our 

 globe and its inhabitants, and can sketch out with confidence many of the 

 changes they must have undergone." 





