68 
5 , If we develope 
d /Bp D dp Bd^ 
dz \dy dx ) dy dz dx dz 
( 6 ) 
we shall see that it is an identity ; for the sinister only dif- 
fers from the dexter in having the terms 
dp dq 
dz dz 
^ whicli-O 
dz dz 
or cancel one another. Hence 
dp j dq 
-~dx ^~~dy 
dz dz 
is a perfect differential, not in general integrahle in its pre- 
sent form. It is in fact the differential of the logarithm of 
the reciprocal of the integrating factor of (2). 
EREATA. 
Page 34, for lines 8, 9, 10, 11, 12, 13 read — I can merely bring before 
you a few of tbe more noteworthy points wliich stand out from my 
general reading, more especially relating to the History of Ancient Life 
on the earth, together with some general considerations which seem to 
me opportune under the circumstances of the time.” 
Page 40, line 4 from the bottom, for flu viable” read fluviatile.” 
Page 40, last line, for ‘^the equivalent of” read ‘"as old as.” 
