1919-] Geographical Distribution and Miyration. 389 



All such distribution, as illustrated in this last paragraph, 

 is due either to gradual emigration or to a regular migratory 

 habit at some remote period, and has depended for its success 

 on tiie initial capacity of a species to adapt itself to new 

 surroundings, which was possibly a case of necessity in the 

 earliest attempt. 



In this connection it would be interesting to ascertain 

 whether the same species, when nesting in tropical countries, 

 lays fewer eggs in the clutch and rears more broods in the 

 season than the same bird in more northern climes, "^riie 

 Blackbird is said (Chapraati, ' Wild Spain ') to lay but three 

 eggs in Spain, to raise throe broods in Tangier (Favier), whilst 

 in the Canaries the local Blackbird {Turdus m. cabrerai) lays 

 very few eggs in the clutch (Ibis, 1912, p. 597). The Wren 

 ( Troglodytes), a prolific breeder in northern climes, appears to 

 lay but four eggs in the normal clutch in Sicily (Ibis, 1912, 

 p. 171). Is such the case among other species which have 

 tropical representatives? The point is submitted to the many 

 distinguished zoologists whose vast collections might help 

 to solve the proljieri). Is the normal clutch regulated by the 

 capacity of the parents to feed the young (or water the young, 

 in the case of Sand-Grouse), or by the limits of brooding-sur- 

 face on the parent's abdomen, or by the normal mortality in 

 the species, or by what ? Even such questions have influence 

 on migration and distribution, for it is by no means certain 

 whether birds go to the Arctic Regions for reproduction, on 

 account of their ancient love for home, or to enable them 

 to get sufHciently long days to collect a satisfactory supply of 

 food for their offspring, or whether merely because the 

 Arctic liegions offer a more prolific food-supply than more 

 southern regions. If either of the two latter causes are 

 correct, we should expect to find the Charadriidsc and 

 Anatidse which breed in the tropics to lay fewer eggs in the 

 clutch than those which breed in northern Europe. We know 

 that a plethora of food reflects itself on reproductiozi {(^. 

 Snowy Owls and liough-legged Buzzards in Lemming-years 

 in Scandinavia, and the increase of Hysenas after wholesale 

 deaths among natives in East Africa). 



