628 AVES [CH. 



seems hardly credible. There is no doubt, however, that very large 

 numbers perish in crossing the sea. 



Remains of fossil birds earlier than the tertiary period are 

 very rare, but a few exceedingly interesting specimens have, 

 however, been obtained. The principal of these is Archaeopteryx, 

 represented by two specimens from the quarries in lithographic 

 stone at Solenhofen in Germany. This remarkable bird had a long 

 tail like that of a lizard, to each vertebra of which a pair of feathers 

 was attached ; the fingers of the wing bore claws and the bones of 

 the palm (metacarpals) were free from one another. In the skull 

 the premaxilla was as usual ensheathed in a horny beak but the 

 maxilla bore teeth. 



In all these points Archaeopteryx maybe said to retain reptilian 

 characters. Two other fossil birds (Hesperornis and Ichthyornis) had 

 teeth in the maxillae but in other respects their structure was like 

 that of modern birds. 



The classification of Birds presents great difficulties. They are 

 a modern and very successful group of animals and 

 evolution in them is proceeding rapidly. Older 

 taxonomists were wont to classify them by the shapes 

 of their beaks and their claws; modern taxonomists are perhaps 

 too apt to reject these as external characters " of a purely adaptive 

 nature." Such comments raise some fundamental questions on 

 which a few words may be said here. If the evolution theory be 

 justified .all characters in animals are adaptive : they have either 

 been directly acquired in response to the demands of the environ- 

 ment, or they are the secondary result of the adaptation of other 

 organs to the environment. Now the thought underlying the dis- 

 trust of external characters as a basis of classification is that they 

 in their form represent the most recent adaptations since they 

 come first into contact with the environment; the more deeply 

 situated organs it is thought, are more slowly affected and hence 

 represent more ancient adaptations, and therefore afford indications 

 of the affinity of animals whose external features have become 

 different. There is a good deal to be said for this principle but 

 it must be used with caution. As Herbert Spencer pointed out 

 long ago, the alimentary canal, the most internal organ of all, 

 comes into close and direct relation to the environment. 



The bones on which systematists are wont to lay what seems to 

 us an overwhelming amount of stress alter quickly and directly 

 according to the development of the muscles which are attached to 



