36 DISPOSITION TO SECURE A TERRITORY 



imagine that, in the course of a journey pro- 

 longed through some 50° or 60° of latitude, 

 the stronger individuals should outstrip the 

 weaker by a very perceptible distance, and it 

 can hardly be doubted that in most species 

 the males are stouter, as they are bigger 

 than the females." Granting that the males 

 are the stronger, how can this account for their 

 outstripping the females by a week, ten days, 

 or even a fortnight, in a journey of perhaps 

 1500 miles ? To expect the birds to accom- 

 plish such a distance in seven days is surely 

 not estimating their capabilities too highly, 

 and any slight inequality in the power of 

 flight or endurance could give the males an 

 advantage of a few hours only. But this ex- 

 planation, based upon inequalities in the power 

 of flight and endurance on the one hand, and 

 the magnitude of the distance traversed on the 

 other, cannot afford a solution of the behaviour 

 of the resident males, and is less likely, there- 

 fore, to be a true solution of that of the 

 migrants. 



There is another theory, simple enough in its 

 way, which will probably occur to many. It is 

 based on the assumption that the males reach 

 sexual maturity before the females ; and it is 

 contended that the functioning of the instincts 

 which contribute towards the biological end of 

 reproduction depend upon the organic changes 

 which the term " sexual maturity " is held to 

 embrace, and that, inasmuch as the migratory 

 instinct belongs to the group of such instincts, 



