THE CARRION-CROW AND THE HOODED-CROW 19 



dog and the wolf, the dog and the jackal, the pintail duck and the 

 mallard, and many others, all of which produce fertile hybrids. It is, 

 therefore, left to the personal judgment of the individual system- 

 atist to determine the quantity and quality of the distinctive 

 characters that may be allowed in any given case to divide one 

 closely related form from another. As systematists are prone to 

 differ, and as the species in many Genera merge into each other 

 through an unbroken series of transitional forms, it is not sur- 

 prising that a confusing variety of opinion is the result. The 

 extent to which this may be carried is illustrated by the well-known 

 example given by Haeckel, who showed that there may be 3, 21, 289, 

 or 591 species of calcareous sponges, according to the point of view 

 from which they are regarded. 



In the case of the hooded- and carrion-crow, besides the difference 

 in coloration already described, there is a notable difference in the 

 minute structure of the shells of their respective eggs, those of the 

 rook and jackdaw being, in fact, much more alike in this respect. 1 



Some writers are of opinion that the two species can further be 

 distinguished by their notes, or some of them, but as the voices of 

 birds vary with the individual, it would require very close observation 

 to establish a distinction which, in any case, cannot be strongly 

 marked. Whether there is any difference between the songs of the 

 two species is difficult to say, for no record of that of the carrion-crow 

 appears to exist, though no doubt the bird has a song, or what may 

 pass for such. The hooded-crow, like the raven, indulges in a con- 

 tinuous succession of varied notes. That the bird is not ashamed of 

 its vocal powers is evident from the fact that the performance usually 

 takes place on the top of some high tree or building, and evidently 

 affords great satisfaction to the performer. 



Whether or not the above-mentioned differences give the two 



1 For the structure of the egg-shells of hoodies, crows, and other Corvidse, see the Journal 

 fur Omithologie, 1874, pp. 1-26 ( W. von Nathusius). For interesting observations on the hybrids 

 of birds, see J. L. Bonhote's articles in the Proceedings of the Fourth International Ornitho- 

 logical Congress, 1905, p. 233, sec. 2, and in the Proceedings of the Zoological Society, 1902 and 1905. 



