July, 1914 A PLEA FOR COMPARATIVE OOLOGY 165 cess of development. Though the process is far too slow for direct observation it is probably exceedingly rapid in comparison with most evolutionary changes. Usually, we may assume, such a change would be gradual, but it might in some cases be sudden and discontinuous. This latter condition would seem to be likely when the eggs of some particular species stand forth conspicuously' as wholly different from those of all nearly related forms (e.g., Cistothorus stellaris). Great similarity between the eggs of birds distantly related is far less com- mon than dissimilarity among forms that are closely allied. Birds as unlike as parrots and petrels may lay eggs which appear indistinguishable, but 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 even 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 but 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 Alaudi- dae and Hirundinidae and it happens that in each of these groups the eggs, as a whole, are very closely allied. Larks' eggs, while difficult to describe, con- form to a type which is quite distinguishable, while all swallows' eggs seem to be 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 be white, or nearly so; R. riparia, I. bicolor, T. thalassina and S. serripennis are all hole-breeders. It is generally assumed that coloration is primarily a protective feature, and that it is lost, as useless, where 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 S. cunicularia are white just because they are owls' 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 be apparently slight and inconsequen- tial should not discourage our attempts to ascertain them; it is far easier to say they are "accidental", but more logical to assume that they follow some law if we can but find it. On the other hand, many efforts have been made to explain such coloration by the application of general principles affecting or: ganic evolution as a whole, but the results of such broad speculations can hardly be expected to answer such minute requirements. In any event they are beyond the purview of this paper, but to the oologist who is sufficiently in- terested I would commend a perusal of Dixon's chapter on "Nidification" in Seebohm's delightful "British Birds". So much for this line of study, which the "comparative oologist" may amplify indefinitely. But other investigations lie invitingly at hand. Con- sider how little we know of the many unusual types of coloration which occa- sionally occur, departures from the mean which are sufficiently marked to be noteworthy and yet which do not fall within the category of "abnormalities", the latter offering a special field of its own which Jacobs at one time culti- vated most successfully. In Europe collectors are particularly keen in the pursuit of "varieties", as these rarer types are called, and specimens have
