198 JOURNAL, BOMBAY NATURAL HISTORY SOCIETY, Vol. XVIII. 



No. XVII.— DIFFERENTIATION OF SPECIES. 



In taking up the subject of Natural History generally, and of that part which 

 deals with birds in particular, the budding naturalist must frequently be 

 puzzled as to the reason why two birds, which resemble each other very 

 closely in almost every detail, should be separated as two distinct species ; 

 and then again, another pair, which even to his uninitiated eye appear quite 

 different, are classed as one and the same bird probably from two different 

 parts of the country. 



In this respect, the Raptores of India, perhaps more than any other family 

 or order, produce the best examples to illustrate my meaning. In many cases 

 the Falconidce undergo vast changes, from the young to the adult plumage, 

 but that is not all, for once a series is obtained showing the varied plumages 

 from the nestling to the twu-year-old and thence to the adult, the problem 

 would be solved if such were the case, and even the novice would be able to 

 see at a glance, by comparing with the series, what he had got, but the trouble 

 comes in when a nestling plumage, even, is not constant, as in Bateo ferox, the 

 Long-legged Buzzard ; or again as often happens, two specimens ot the same 

 species are procured, presumably of the same age, but in one case the specimen 

 has undergone a perfect moult into the next dress, whereas the other only a 

 partial one and still retains many feathers of the previous year's plumage, and 

 thus the two would present quite a different appearance to each other. 



To return, however, to my original discussion, i.e., the similarity of two 

 distinct species to one another and theu again the difference in two others of 

 the same species. If each of these local changes were to be given generic or 

 specific rank, there would, absolutely, be no end to the numbers, but then again 

 the question crops up, where and how is the line to be drawn ? As an example 

 of the variable types of one species, let us pick out Spilornis cheela, the 

 Crested Serpent Eagle. If each variety of this eagle to be found in India was 

 to be classed as a distinct species, as it formerly was, wo should have at least 

 five extra species in this one alone. Apart. from the difference in size between 

 the Northern India type and that of Ceylon and Southern India, we find vast 

 changes also in plumage. To take the typical form of S. cheela as found in N. 

 India and the Himalayas, we find the following characteristics : Breast and 

 fore-neck distinctly barred ; throat black, and in adults a single white bar on 

 the tail (Blanford). Another, the common type found throughout India, 

 known as S. melanotic, has either none, or very indistinct barring on the 

 breast ; very rarely any black on the throat and frequently two white bars on 

 the tail. Another form (S. rutherfordi) has the barring on the breast as in 

 typical S. cheela, but the throat dark ashy gray. Besides these there are 

 other varieties differing in size and in plumage, passing from 01 e pbase to the 

 other, till in parts there is no distinguishing one from the <>ther, and this 

 being the case, all these have been incorporated under the one main species 

 Spilornis cheela, though the difference in size in some forms is as much as 

 six or seven inches in length, besides the varieties of actual markings above 



