Vol. xxxvii.] 26 



throughout the year in tropical and semitropical countries, 

 but this plenty is found more or less at all times and 

 seasons, and the energy which impels procreation is never 

 in danger of complete exhaustion, whilst at the same time 

 it never reaches the concentrated height it does in countries 

 in Avhich, tor a great part of the year, continued effort is 

 needed for self-support, so that there is no surplus energy 

 available for the production and bringing up of a family. 



" In the Arctic Circle and countries adjaceiit we have a 

 short, hot summer, during which insect and certain forms 

 of ve;;etable life are most extraordinarily abundant : birds 

 feed freely and with little exertion, and attain a super- 

 vitalized condition, with the contingent result that we have 

 bigger clutches of eggs laid by them than are laid by indi- 

 duals of the same species in more temperate countries. 

 But we must remember this also : if we have larger 

 clutches, due to the short fevered summer, we also have 

 fewer clutches laid by the same female, for, once the season 

 is fairly advanced, there is not much chance of her finding 

 time to lay a second. First clutches which are destroyed 

 can often never be replaced ; therefore Nature demands larger 

 clutches to balance the limitation in the breeding-season. 



" When we get into tropical countries we find the reverse 

 process is in existence. The breeding-season is of much 

 greater length, and many birds breed off and on practically 

 throughout the year. Numerically large clutches are 

 unnecessary, for if they are destroyed they can be replaced, 

 and even if the second is lost, yet a third is possible, for 

 food in plenty is obtainable and no excessive exhaustion is 

 entailed in procuring it. 



" But though in the Tropics this rule of small clutches 

 holds good, yet in such countries local conditions cause 

 local and temporary variations far more marked than in 

 the higher latitudes. Thus a long drought may dry up 

 marshes and rivers in the vicinity of which water-birds 

 normally breed in thousands. With no water there are no 

 fish, the birds are underfed, poor in vitality, and with 

 no surplus energy. In consequence, they either do net 

 breed at all or make a short migration to some adjacent 



