DEVELOPMENT OF THE CHICK. 68x 
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 1090 
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 ovayian ovum of the hen is a large spherical body, consisting 
chietly 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 
