EEED WAEBLBR 



be eliminated just as surely as one whose members did not 

 secure sufficient ground to ensure an ample supply of food for 

 their young. But it is clearly impossible to trace the history 

 of the development of the breeding territory unless we have 

 before us a true phylogeny, and the conditions under which 

 the earlier types existed. Many of the sea-birds, which breed 

 upon ledges of rock, are compelled to travel some distance 

 before they can find a situation suitable for rearing their 

 offspring, whereas the question of food presents no great diffi- 

 culty ; consequently the limits of their territory have been 

 reduced to the minimum size necessary for reproduction. In 

 a case in which the territory has been developed in order to 

 ensure an ample food supply during the period of reproduction, 

 it is possible that the area of that territory, and consequently 

 the numerical strength of the species, will be found to be 

 proportionate to the distance its members are accustomed to 

 travel in search of food. This, in some instances at least, 

 appears to be the case. Those species that have long dis- 

 tances to travel are represented by comparatively few, and 

 conversely those that find food close at hand by numerous 

 members. The birds of prey wander over a great distance, 

 consequently their territories are large, and the same may be 

 said of the Eaven. It is not every tree that can supply the 

 necessary food for the Woodpecker — in fact, such trees are 

 relatively scarce — therefore it has first to search for and 

 find the suitable trees, and then it will visit them daily, 

 generally in routine, covering a considerable area while so 

 doing. At the same time it by no means exhausts all the 

 possible food supply ; the area covered would possibly be 

 sufficient to maintain one other pair at least, but by reason 

 of the fact that the food is difficult to find the distance 

 traversed may be beyond what is really necessary. In such 

 a case the conditions of existence might be so strenuous that 

 it would become of the utmost importance to any particular 

 individual to prevent, not only members of its own species 

 but also those of closely allied species, from breeding in the 



31 



