622 BIHDS. 



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 solely 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 some 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, some 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 

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

 plasm with a large nucleus. The ripening of the egg is accompanied 

 by the disappearance of the nuclear membrane, and also by the forma- 

 tion 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 becomes 

 surrounded by various envelopes added to the delicate vitelline membrane 

 with which it is already invested ; on the other hand, segmentation goes 

 on rapidly in the formative area. 



The fully formed and laid egg is surrounded by a firm porous shell of 

 carbonate of lime, and beneath this there is a double shell membrane, 

 the two layers of which are separated at the broad end of the shell to 

 form an air-chamber. This chamber grows larger as development pro- 

 ceeds, and is of some importance in connection with respiration, as an 



