28 



THE GLACIAL EPOCH. 



Interglacial 

 periods not 

 affeoting the 

 Poles. 



Fossil re- 

 mains of 

 birds. 



Causes of 

 their rarity. 



Probability 

 of an Eocene 

 Glacial 

 Epoch. 



to believe that the ice never left the Polar Basin during the last Glacial Epoch, though its 

 southern range ebbed and flowed with the precession of the equinox, causing great altera- 

 tions of climate in subarctic regions. This theory is supported, as far as it goes, by the 

 evidence derived from a study of the geographical distribution of the Charadriidee. 



The facts to be learned by a study of fossil birds are very few. Fossil birds are rare 

 and very fragmentary. The fossil remains of mammals are often found in caves, where 

 they had taken refuge, and where they were drowned during floods, and their bones 

 preserved in the mud and sand washed in by the water. Other fossil remains of mammals 

 have been deposited in the mud at the mouths of rivers. It is obvious, however, that 

 neither of these accidents, if we may so call them, can often happen to birds. Very few 

 species would be likely to seek refuge in a cave, nor would their dead bodies be likely to 

 sink in the bed of a river. The air-cavities in the bones of a bird make it so light that it 

 must float on the surface of the water and eventually be washed ashore, when it naturally 

 falls a prey to one of the carrion-eating beasts or birds, which are the scavengers of the 

 coast. The fossilization of a bird is a comparatively rare event, and their remains few and 

 fragmentary. It seems probable that traces of birds are found in the Triassic formation ; 

 no actual remains of birds have been found, but footprints, which appear to be those of 

 birds, occur. The celebrated Archseopteryx was found in Bavaria in a limestone-formation 

 belonging to the Jurassic period. Like the majority of the birds of the Cretaceous fauna, 

 the Archseopteryx shows its affinity to its semireptile ancestors by its teeth. These toothed 

 birds appear to have become extinct before the beginning of the Tertiary Period ; but 

 Milne-Edwards, in his treatise on the Miocene fossils of France, comes to the conclusion 

 that some of the fragments of fossil skeletons of birds must be referred to existing Limico- 

 line genera, such as Numeniiis, Tringa, Totaniis, and Himantopus, We must therefore look 

 to a period which may be the Glacial Epoch, of which there appears to be some geological 

 evidence in Eocene times \ and which may possibly have been caused by the remarkable 

 increase in the eccentricity of the earth's orbit, which CroU shows as having occurred from 

 eight to nine hundred thousand years ago, as the time when the ancestors of the present 

 Charadriine genera were diff'erentiated, and previous to which they formed one species, living 

 on the shores of the Polar Basin. 



It is probably impossible to form anything more than a wild guess at the date of these 

 wonderful phenomena, but we may be reasonably allowed to believe that they occurred, and 

 that the effect of their occurrence was the differentiation of Charadriine birds, first into 

 genera, and secondly into species: so that, in this family at least, the genera are not 

 absolutely artificial, but are to a large extent the impress upon this group of birds of great 

 geological events. 



In order to make the narrative intelligible, I propose to call the last Glacial Epoch, 



' " With respect to more ancient Glacial periods, several geologists are convinced from direct evidence 

 that such occurred during the Miocene and Eocene formations." (Darwin, ' Origin of Species,' 5th ed. p. 452.) 



