ENVIRONMENT, PAST AND PRESENT 67 



The first toothless birds make their appearance 

 in the upper Cretaceous and the first truly modern 

 birds in the Eocene. The fossil series of birds is, 

 unfortunately, very badly broken and there are 

 still enormous gaps in the record. 



Before leaving the subject it might be stated that 

 the gauging of the ages of the epochs is not a matter 

 of guess-work, but is based on various reliable 

 methods such, for instance, as the rate of sedimenta- 

 tion on the ocean-floors and the thickness of the 

 various rock-strata, or the relative amounts of lead 

 and radio-active substances in various minerals, and 

 it seems quite certain that birds have a distinctly 

 lengthy pedigree. 



While we propose to make a brief excursion into 

 the climates of the past, back to the days oi Archaeop- 

 teryx, such a digression is probably of no great sig- 

 nificance, for whatever may have been the state of 

 the evolution of migrations prior to the last ice-age — 

 and northern migrations almost certainly go back 

 to the early Pliocene, i.e. before the ice-ages — almost 

 the entire story must have been retold for most 

 species on this continent after the last retreat of the 

 northern ice-sheet. There is no reason to imagine 

 that the factors at work either before the ice-age or 

 immediately after it differ essentially from those in 

 operation today and it would seem possible to iden- 



