July, 1914 
A PLEA FOR COMPARATIVE OOLOGY 
165 
cess of development. Though the process is far too slow for direct ohservation 
it is probably exceedingly raiiid in comparison with most evolutionary changes. 
Usually, we may assume, such a change would be gradual, hut it might in some 
cases be sudden and discontinuous. This latter condition would seem to he 
likeh' when the eggs of some particular species stand forth conspicuously as 
wholly different from those of all nearly related forms (e. g., Cistotliorus 
stellaris). * 
Great similarity between the eggs of birds distantly related is far less com- 
mon than dissimilarity among forms that are closely allied. Girds as unlike 
as parrots and petrels may lay eggs which appear indistinguishable, hut this 
is due to lack of color in each case. In fact, unless eggs are white or, at most, 
plain colored, family distinctions usually prevail, and this holds generally evei: 
in the Oscines where natural lines are faintly drawn. Of course this does not 
imply that the eggs of each family necessarily show any great similarity l)ut 
rather that eggs selected from different families are usually sufficiently unlike 
to prevent confusion. Nevertheless, of the fifty (more or less) oscinine fami- 
lies the two which, I presume, are the most clearly delimited are the Alaiidi- 
(lae and Jlirundinidac and it happens that in each of these groups the eggs, as 
a Avhole, are very closely allied. Larks’ eggs, while difficult to describe, con- 
form to a type which is quite distinguishable, Avhile all swallows’ eggs seem to 
he white, some more or less flecked with brown. Further investigation of the 
varous swallows’ eggs illustrates one principle in oology which is fairly con- 
stant, namely, that eggs hidden in holes are apt to he white, or nearly so ; 
7?. riparia, I. bicolor, T. thalassina and 8. serripennis are all hole-breeders. It 
is generally assumed that coloration is primarily a protective feature, and that 
it is lost, as useless, Avhere eggs are completely hidden from view. Unfortun- 
ately there are also plenty of white eggs laid in open places: the eggs of both 
Asio accipitrinus and »Sf. cunicidaria are white .just because they are ou'ls’ eggs, 
in all probability, irrespective of the fact that one bird exposes its eggs on the 
ground while the other burrows beneath it. At all events, the production and 
deposition of egg coloring matter must correspond to certain definite physi- 
ological, chemical and, perhaps, anatomical characteristics in one or both par- 
ents, and the fact that these causes may he apparently slight and inconsequen- 
tial should not discourage our attempts to ascertain them ; it is far easier to 
say they are “accidental”, hut more logical to assume that they follow some 
hnv if Ave can hut find it. On the other hand, many efforts haAm been made to 
explain such coloration by the application of general principles affecting or- 
ganic evolution as a Avhole, hut the results of such broad speculations can 
hardly he expected to ansAver such minute requirements. In any event they 
are beyond the puiwieAv of this paper, but to the oolpgist Avho is sufficiently in- 
terested I Avould commend a perusal of Dixon’s chapter on “Nidification” in 
Seehohm’s delightful “British Birds”. 
So much for this line of study, AA^hich the “comparative oologist” may 
amplify indefinitely. But other investigations lie invitingly at hand. Con- 
sider hoAv little Ave knoAV of the many unusual types of coloration AA'hich occa- 
sionally occur, departures from the mean Avhich are sufficiently marked to ho 
iioteAvorthy and yet Avhieh do not fall Avithin the category of “abnormalities”, 
the latter offering a special field of its OAvn which Jacobs at one time culti- 
vated most successfully. In Europe collectors are particularly keen in the 
pui'suit of “varieties”, as these rarer types are called, and specimens have 
