54 THE MIGRATION OF BIRDS. 



Climatal Change, if it lias been enormous and severe, 

 has been gradual. We should naturally expect to 

 find therefore the result of this reflected in present 

 Migration. This is undoubtedly the case ; for 

 Migration is gradated in many various degrees 

 both of space and time. It is by no means uni- 

 versally extended among species, or even, as we 

 have already seen, among individuals of the same 

 species {conf. p. 20). The flights were short at 

 first, and were very gradually extended ; varied 

 likewise as to time and performance, as the seasons 

 became shorter or longer. The evidence for this 

 is abundant, convincing, and indisputable. The 

 irregular appearance of many northern species in 

 such southern latitudes as the British Islands, 

 during exceptionally severe winters, admirably illus- 

 trates how under more pronounced conditions of per- 

 sistent cold, Migration could be initiated. It is easy 

 to understand how, if the winters were gradually to 

 become uniformly and constantly more and more 

 severe, these species, instead of paying only occasional 

 and irregular visits south, would in the course of 

 ages become regular migrants ; and the habit or im- 

 pulse to migrate, owing to the laws of heredity and 

 a continual climatal cause, w^ould become finally a 

 deeply-rooted one. There can be no doubt what- 

 ever that this is very similar to wdiat has actually taken 

 place, not only throughout the varying climates of 

 Prae-Pliocene ages, but during the low^ering temper- 

 ature of the Pliocene period itself, which, according 

 to Professor Marcou and other scientists, was actually 



