776 NATURAL SCIENCE. dec. 



knots which pass along the West Coast of Europe extends from 

 Southern Scandinavia, along the British and Continental coast-lines, 

 to the south of Africa, wintering wherever the conditions are favour- 

 able along that line for the support of life. Several instances could 

 be adduced to show that an abnormal supply of food in a special 

 locality has the effect of staying altogether or retarding the passage 

 and range of species which, under normal conditions, would have 

 been more extended. The extent of the longitudinal migration of a 

 bird is the affair of the individual, not of the whole race, moving from 

 one extreme of the range to the other. Therefore, it appears to be 

 impossible to draw, as Mr. Dixon attempts to do, deductions as to 

 short or long migratory flights correlated with ancient or more 

 recent geological epochs. 



There are certain general laws of migration accepted by ornitho- 

 logists ; these may be briefly stated, (i) Each bird (excepting 

 purely tropical species) breeds in the coolest, that is the most 

 northerly, part of its range, and that ^hese nesting quarters are 

 reached by horizontal migration. Some species and individuals, 

 however, obtain the necessary conditions for reproduction in a 

 vertical migration by ascending mountains till they meet with the 

 same climatic conditions which exist in high latitudes. (2) North- 

 ward migration is for reproduction, southward for food and climate. 

 (3) Birds which go furthest north to breed often retire the furthest 

 south. (4) Each species has its regular periods of migration. (5) 

 No bird is known to breed a second time during absence from its 

 northern home. 



The lines of migration, or bird routes, in the east and west 

 Palaearctic and Nearctic regions are infinite and differ in the case of 

 each species and even with individuals. The general direction is 

 from north to south, but there are many variations apparently 

 running in very erratic directions; the bird lines also vary from time 

 to time under the influence of the weather. There are also certain 

 routes or great highways where the migratory host focusses its 

 millions into streams. Such is the line across Heligoland and along 

 the west coast of Europe. There are also many facts suggestive of 

 birds continuing to follow persistently ancient land-lines, now 

 submerged; and we know of no species or individuals which systema- 

 tically cross seas or oceans where land has not probably existed in 

 recent or more remote geological ages, and since the habit of migration 

 has been evolved. 



There is evidence of ancient lines of migration still followed by 

 a few species between India and South Africa, also by Eastern Asia 

 to New Zealand, and from the western coast of North America to the 

 Isles of the Pacific, routes which, probably at some remote period, 

 offered greater facilities over a more continuous land surface. 

 Evidences of more recent land routes are suggested across the North 

 Sea to the British Islands, and over the Mediterranean by Sicily and 



