MUTATIONS AND EVOLUTION, -^qQ 



Stages can be prLKHired from three rows, to where as many a^ 

 ten plumes are wanting from the elbow end of the single first 

 row and four at the other end. The loss of plumes follows a 

 regular ordnial succession; but prior to its final disappearance 

 each plume passes through a definite series of retrogressive 

 stages, represented by a gradual diminution in size and then loss 

 of constituent parts. Observation shows that usually only one 

 or two plumes at the end of a row are in a degenerative phase 

 at the same time. The valuable remiges fluctuate from 33 

 to 44, diminutive or vestigial feathers sometimes occurring at the 

 retrogressive end of the row similar to those in the rows of 

 coverts. The first row of upper-coverts has barely started on 

 its course of degeneration, but the second fluctuates from a 

 complete row to one where as many as half have disappeared, and 

 the succeeding rows in some birds are greatly reduced in length 

 and number compared with others. 



Down feathers are usually stated to be absent from the 

 Ratita. but in the ostrich they are still to be found as survivals 

 around the base of the larger plumes of the wings and tail, 

 and individuals occur with vestigial down over the hind part of 

 the body and even under the wings. Moreover, all stages in 

 the loss of the individual feather, represented first by diminution 

 and then by loss of ccnstituent parts, are presented, similar to 

 those exhibited by the coverts and remiges. 



All intermediate stages occur in the degeneration of the 

 second phalanx of the third digit of the wing, from a completely 

 free bone forming a projecting free third digit to a mere nodule 

 fused to the end of the first phalanx, and not apparent from the 

 surface. The digit is also unique among birds, in that it 

 occasionally bears its own plumes, like the ala spuria. Traces 

 of four metatarsals occur in the embryo chick with a ]:>halanx 

 to the second reminiscent of the three-toed ancestor, and all 

 stages can be i^rocured in the loss of the claw from the small 

 fourth toe, and also in the disappearance of scales from the 

 big middle toe, showing that the latter has also started its 

 course of degeneration. While in the main the various retro- 

 gressive stages show Mendelian proportions in breeding 

 experiments, departures from this seem to find their interpretation 

 in the fact that losses are actually in progress. This is particu- 

 larly the case as regards the nail on the fourth toe and the 

 scales on the third, where the progeny of crosses usually show 

 much reduction compared with the parents. In all cases a 

 definite succession in diminution in number or size or loss of 

 structural parts is indicated, as compared with one haphazard or 

 irregular, one loss preparing the way for the next, and rendering 

 it possible. 



The many breeding experiments have shewn that all the 

 variations are germinal in their origin, as distinct from somatic 

 fluctuations. None of the birds are yet germinally pure as 



