DEVELOPMENT OF THE CHICK. 681 



of glacial conditions from the north would gradually force birds, century, 

 by century, to a longer flight southwards. And if the climatic condi- 

 tions limit the area of safe and comfortable breeding to one country (the 

 more northerly), and the possibility of food during winter to another 

 country (the more southerly), we can understand, with Wallace, "that 

 those birds which do not leave the breeding area at the proper season 

 will suffer, and ultimately become extinct ; which will also be the fate 

 of those which do not leave the feeding area at the proper time. " In 

 short, given environmental changes of climate on the one hand, and a 

 measure of plasticity and initiative on the part of the organism, the instinct 

 of migrating would be perfected in the course of natural elimination. 



But while this view is so far satisfactory, it leaves us face to face with 

 the problem how birds migrate as safely and surely as they do on their 

 pathless way. For to point out that the merciless elimination which 

 continually goes on keeps up the standard of racial fitness, leaves us 

 still wondering how any ^became fit at all. 



One welcomes therefore any suggestion as to the manner in which 

 birds learn or have learned to find their way. The power has been 

 compared to the " homing " faculty of some pigeons, but most believe 

 that pigeons are guided largely by noticing landmarks, which could 

 hardly be done over 10,000 miles of land, and obviously not over 1000 

 miles of sea, or during the night. Some have urged that birds follow 

 river valleys, the lines of old "land bridges" connecting continents, 

 the roll of the waves, and so forth, but the difficulty remains of flight by 

 night and at very great heights. Attractive is the suggestion that birds 

 are guided by what may be called a " tradition " based on experience ; 

 those guide well one year who have followed well in previous years. 

 But many young birds fly apart from their parents, and some birds do 

 not fly in flocks at all. Moreover, it is difficult to understand how the 

 experience could be gained except by sight, which in many cases is 

 excluded by the darkness. In face of these difficulties, many authorities, 

 such as Professor Newton, have been led to believe that birds have, in 

 an unusual degree, " a sense of direction." 



DEVELOPMENT OF THE CHICK 



The ovarian ovum of the hen is a large spherical body, consisting 

 chiefly of yolk, but exhibiting at one region a disc of formative proto- 

 plasm with a large nucleus. The ripe ovarian egg is surrounded by a 

 vitelline capsule, mainly due to the follicular theca in which it is formed. 

 There is an innermost non-cellular membrane, then an epithelium, then 

 a connective tissue outer membrane. The ripening of the egg is 

 accompanied by the disappearance of the nuclear membrane, and also 

 by the formation of polar bodies ; but the details of the process are 

 obscure. 



Either before it leaves the ovary, or in the upper part of the oviduct, 

 the egg is fertilised by a spermatozoon. During its passage down the 

 oviduct it undergoes two sets of changes. On the one hand it is sur- 

 rounded by various envelopes added to the delicate vitelline membrane 



45 



