EEED WARBLER 



tinuous. Thus the range would be gradually extended further 

 and further from its base until it began to advance into 

 regions subject to alternations in climate, and in which, con- 

 sequently, the duration of food supply was more or less limited 

 to a few months in the year. Whenever this point was 

 reached, it would be necessary for the individuals to return 

 for part of the year within the zone of perpetual food supply. 

 Now we know that, in the case of the migratory species, there 

 is a strong propensity in the individual to revisit in the follow- 

 ing breeding season that particular neighbourhood in which it 

 was reared. But even if we did not know this we should infer 

 that it was so from our knowledge of the conditions under 

 which the range of the species is being extended, i.e., from the 

 law of territory. For if those males that were forced to desert 

 a certain district through insufficient food supply during part 

 of the year did not again go forward the following spring to 

 their former breeding haunts, overcrowding would occur at 

 some point ; and inasmuch as reproduction would be impos- 

 sible to a large and increasing number, owing to lack of terri- 

 tory, the distribution would remain stationary, and a check 

 would be administered to the welfare of the species. There- 

 fore any variation in the direction of a tendency to return 

 would be fostered and developed by selection. And so it 

 would be with the females ; those that did not return to their 

 birthplace, or their old breeding haunts, would be less likely to 

 reproduce, since they would have greater difficulty in finding 

 an unpaired male in possession of a territory. We can then 

 understand that the range of our imaginary species would be 

 extended with a corresponding increase in the distance 

 traversed by some of its individuals between the zone of 

 perpetual food supply and the breeding haunts, until the limit 

 imposed by unfavourable conditions of existence were reached. 

 And when we consider the conditions under which it must 

 have existed, the constant struggle and constant search for 

 new breeding grounds, changes in the earth's surface, and 

 changes of climate, resulting in the gradual disappearance of 



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