40 PHILOSOPHY OF ZOOLOGY. 



The extent of degrees of latitude traversed in these migra- 

 tions, differs, as we have seen, according to the species, and 

 even in the same species in different parallels of longitude. 

 Thus, the nightingale, in its polar migrations, does not reach 

 the 55 of north latitude in Britain, while in Sweden it reaches 

 to the 60. Anomalies of this kind cease to excite our sur* 

 prise, when it is considered, that the isothermal lines (or the 

 latitudinal lines under which the mean annual temperature is 

 the same), are not parallel with the sun's course, or do not 

 observe a regular increase or diminution with the difference 

 of latitude. Even the isotJierial tines, (under which the 

 mean heat of summer is the same), and the isotheimal lines*, 

 (under which the mean heat of winter is the same), are neither 

 parallel to one another, nor to the isothermal lines. These 

 differences, which HUMBOLDT has investigated with so much 

 success, exercise a powerful influence on the distribution of 

 plants and animals, and regulate the limits of those periodi- 

 cal migrations we have now been considering. 



The preceding remarks relate to the equatorial and north 

 polar migrations. Movements depending on the same cir- 

 cumstances, in all probability, take place on the other side 

 of the equator towards the south pole. The Cape swallow 

 (Hirundo Capensis), according to the observations of Cap- 

 tain CARMICHAEL, arrives at the southern extremity of 

 Africa in the month of September, the commencement of 

 the summer of that district, and departs again in March or 

 April, on the approach of winter *. Reasoning from the 

 analogies of the north polar migrations, we may conclude, 

 that this species of swallow resides the remaining part of 

 the year near the equator, and that its south polar migra- 

 fions extpnd to the Cape of Good Hope. 



* Edinburgh Phil. Journ. vol. -i. p. 421, 



