4642 Tue ZooLocist—OcToBER, 1875. 
might almost say—as it is that this should be the solution of the 
mystery, we have really nothing that can be urged against it except 
its seeming impossibility—no observed facts, which will contravene 
that or any other hypothesis, however wild. Herr Palmén supposes 
that “experience” is the key—i.e., that flocks of migrants are always 
led by birds which have made the journey before. But experience 
here means a knowledge of landmarks, gained by sight; and how 
then is this to be of use to birds which travel by night, or cross at 
one stretch 1000 miles of sea or land? and it is almost undoubted 
that many birds do so. I cannot conceive a problem more worthy 
of the attention of ornithologists than this, and I suspect that 
ornithologists before they solve it will require the aid of those that 
follow other branches of science. 
Then again there is another subject which has been almost 
entirely forgotten of late years. This is the investigation of what 
have been called the laws of plumage. Here and there we find 
some incidental remarks on the moulting of some particular species, 
but these are often made in support of some preconceived theory, 
and very frequently in total opposition to what has been actually 
observed ; and nowhere that I know of is there any connected series 
of observations on the moulting of birds recorded. This is a subject 
which may perhaps be more profitably investigated by those who 
have constant access to zoological gardens than by field-naturalists ; 
but still it unquestionably comes into the department of observa- 
tional Ornithology, and those who keep tame birds may afford 
efficient aid in this interesting matter. 
Again, too, the period of incubation is a subject of which, in all 
but a very few of our native birds, we are quite ignorant. When 
we find that two species so very nearly allied as the pheasant and 
the barndoor-fow] differ by a week in the time each takes to hatch 
her eggs, without our being able to assign any reason for the dif- 
ference, it shows that there must be something of importance which 
has hitherto escaped our scrutiny, and as such it deserves attention. 
We know not for certain anything at all as to the effects of atmo- 
spheric temperature on the development of the chick. Poultry- 
keepers commonly suppose that, if the weather be warm, their broods 
will come out some hours—it may be a day or two—sooner than if 
it be cold; but who has ever instituted any series of experiments 
that will decide the matter, or determine the conditions of variability, 
if variability there be? The species of birds which breed in Britain 
