CHAPTER XVIII 



NOTES ON THE DEVELOPMENT OF THE JACANA 



An examination of several stages of development in 

 the growth of the jacana brings to light many interesting 

 and perhaps significant facts that so far have been over- 

 looked. There are several curious external characters which 

 cause the adult bird to stand apart from others as an ex- 

 ample of great specialization. These are chiefly due to a 

 gradual change of conditions which called for a development 

 of certain characters and a degeneration or loss of others. 

 The enormous toes and claws enable it, one might almost 

 say, to walk on the waters, and the great shield on the fore- 

 head must have some special use which is still unexplained. 

 The claw of the thumb has degenerated to a mere remnant 

 and that on the finger has quite disappeared, while a huge 

 spur has developed at the wrist and doubtless is of great 

 value as a means of defense. By this interchange of char- 

 acters the bird has become fitted to its present environment 

 so that it stands out above many others as an example of 

 adaptation. In the embryo we find many characters which 

 have long since disappeared from the adult, and in the grow- 

 ing chicks we note the development of many that have been 

 more recently acquired. 



PTERYLOSIS 



In the half developed embryo the feather tracts are in- 

 dicated by elongated papillae, which in reality are developed 

 feather sheaths, and in which may be seen traces of pig- 

 ment. Those of the tail are the most advanced, the sheath 

 buds of the rectrices and their coverts being nearly a milli- 

 meter long and full of pigment. On the flank, running from 

 just above the knee and directly in line with the rectrices. 



