334 



THE RISE OF ANIMAL LIFE 



southward to regions where food is abun- 

 dant and the dimate more comfortable. 

 Most humans would certainly enjoy such an 

 arrangement. All efforts to show that the 

 migration south is started by a sudden drop 

 in temperature have been to no avail. The 

 duck hunter will attest to the fact tliat his 

 quarry will often leave the swamps during 

 a spell of warm weather, whereas at other 

 times, even in the face of an unseasonal 

 snow flurry, they will not budge from the 

 swamps in which they were reared. 



Experiments have shown rather definitely 

 that migrations start at remarkably regular 

 periods which can be determined only by 

 the amount of light received by the birds. It 

 has been found, for example, that birds will 

 start to lay eggs even in the winter time if 

 the length of daylight is increased artificially. 

 The farmer has profited from this informa- 

 tion by turning on lights in the chicken 

 houses, thus stimulating the hens to lay 

 more eggs. Light shining on the retina 

 of the chicken apparently stimulates the 

 pituitary gland (a ductless gland which 

 secretes a number of hormones ) , which then 

 activates the ovary to produce eggs out of 

 season. It hardly seems fair, does it? 



The paths taken by some birds in their 

 migrations are truly amazing and that of 

 the American golden plover is especially 

 so. It nests in June on the northwestern 

 shores of Canada above the Arctic Circle; 

 shortly after the young birds leave the nest, 

 the adults desert them and start across 

 Canada, flying eastward to Labrador where 

 they arrive in early autumn. From there 

 they take a direct course south, flying out 

 over the Atlantic Ocean until they reach 

 Venezuela; they then fly more leisurely on 

 to the region of Paraguay where they over- 

 winter. Another remarkable point is that 

 the young birds follow a month later over 

 approximately the same path, a total dis- 

 tance of over 6,000 miles, with no experi- 

 enced birds to guide them. The answer to 

 this mystery will be exciting, if and when 

 it is found. 



The first birds 



It would be amiss not to mention some of 

 the ancient birds that have been unearthed 

 with the other animal fossils. As has already 

 been mentioned, the birds apparently stem 

 from reptflian ancestors (Fig. 13-41), a fact 

 that should reveal fruitful intermediate 

 types, animals that are neither reptile nor 

 bird but a combination of both. Arcfiaeop- 

 teryx, an ancient bird about the size of a 

 crow, was found in limestone about 75 

 years ago in Germany. Strangely enough, in 

 all of the subsequent diggings no other 

 remains have been discovered. These re- 

 mains, however, were sufficiently intact to 

 describe the bird rather well, even the 

 feathers being preserved. Aside from them, 

 however, the other parts of the animal ap- 

 pear very similar to a small dinosaur. The 

 wings were weak and the finger tips bore 

 claws. The beak was lined with teeth and 

 the skeleton was made up of heavy bones, 

 in contrast to the light, hollow bones of 

 modern birds. The fact that the bones were 

 heavy and that the keel was poorly de- 

 veloped indicates the bird was a poor flyer. 

 In fact, if it could have been observed at 

 that time it undoubtedly would have been 

 seen to glide from branch to branch of trees 

 much the same as the flying squirrel does 

 today. 



Some of the marine rocks of Kansas re- 

 veal later birds. One, the "fishbird" (Ich- 

 thyornis), shows considerable advances 

 over Archaeopteryx in that it possessed a 

 large keel bone, indicating excellent powers 

 of flight. It did retain a toothed beak, how- 

 ever. Aside from this feature it probably 

 was not greatly different from the modern 

 tern. 



Some giant wingless birds 



There are several species of wingless 

 birds on earth today as there apparently 

 were over a long period of geologic time 

 also (Fig. 13-54). These birds are all much 

 alike; they are usually very large, some 



