March 25, 1920] 



NATURE 



10: 



form and production, great variations in the 

 minutiae of feather structure were encountered, all 

 of which have a big- industrial bearing. Scarcely 

 any two birds produced plumage alike in character, 

 and the whole object of the breeder has been to 

 bring together in a single plume the best of all the 

 characteristics distributed among the original wild 

 stocks. Without any knowledge of Mendelism or 

 of the factorial hypothesis, the ostrich farmer has 

 clearly grasped the genetic distinctness of the in- 

 numerable " points " of the plume, and the impos- 

 sibility of procuring those desired except from 

 birds already exhibiting them. The best plumage 

 birds in South Africa to-day are the product of 

 only two or three original strains ; all the rest have 

 been discarded as breeders, through not showing 

 sufficient merit. No new character or mutation in 

 plumage has ever occurred since domestication. 

 Selection in breeding is based on plumage alone, 

 no bodily characters having been found to be 

 correlated with it. It is manifest that any ostrich 

 of weak metabolic activity would necessarily pro- 

 duce a defective plume also and be discarded on 

 performance alone, however desirable the pliime 

 factors of the germ might be known to be ; henCe 

 the farmer is more justified in selecting his 

 breeders on production than he would be in select"- 

 ing on pedigree alone. It is not much that the 

 geneticist can do for the practical breeder in cases 

 of this kind ; he can, however, expound to him 

 the soundness of the principles on which he is 

 working and thereby encourage him in his efforts. 



Along with the other members of the ratite 

 group, the ostrich has long been regarded as in 

 many respects degenerate or as undergoing retro- 

 gressive evolution. The relative smallness of the 

 wings and the presence of only two toes to the 

 foot are manifest features, but a closer study 

 reveals many other directions in which loss has 

 taken place, particularly in connection with the 

 epidermal derivatives, scales, feathers, and claws. 

 The thousands of specimens available provide 

 ample material for observing the various stages 

 in the process and the manner in which the loss 

 proceeds. In such studies it becomes important 

 to distinguish between diminution in size and the 

 loss of constituent parts of a structure. Thus, 

 although the wings are so disproportionately small, 

 structurally they are actually less degenerate than 

 in any other living bird. The first and second 

 digits bear claws, and the third digit has some- 

 times a free second phalanx and may bear feathers. 

 The outer toe of the foot is far less in size than 

 would be expected of the fourth in the sauropsidan 

 sequence, yet it retains all its five phalanges. 

 Also, as showing the independence of the de- 

 generative changes one of another, it may be 

 observed that, though the wing is structurally less 

 reduced than in other birds, the foot is unique in 

 having only two toes ; it is more degenerate than 

 in any other living bird. 



That a type has undergone degeneration in any 



respect can be established only by comparison with 



closely related members of the group to which it 



belongs, comparative anatomy affording us a safe 



NO. 2630, VOL. 105] 



standard. On this basis there can be no question 

 of the various lines of degeneration represented 

 in the ostrich, and if among the multitude of 

 specimens examined differences of degree are met 

 with along these lines, it is a fair inference that 

 they represent the various stages of the process, 

 and reveal to us the manner in which evolution 

 proceeds. Whether the occurrence of these inter- 

 mediate stages within extremes proves that evolu- 

 tion is actually in progress to-day may be a 

 reasonable inference in the case of such an animal 

 as the ostrich ; but, as Prof. Bateson has pointed 

 out, it can be definitely established only by com- 

 parisons at long intervals of time showing a 

 general average reduction. 



Comparing, then, the various stages in the de- 

 generation of any particular feature of the ostrich, 

 it is found that wherever a sufficient number of 



Fig. 2. — Series showing stages in degeneration of a feather. 



individuals can be got together a continuous series 

 is presented, linking up the extremes (Fig. 2). 

 Thus birds are to be found with wing quills vary- 

 ing in number all the way from 44 to 33 ; the 

 under-covering of down may be practically lack- 

 ing, while all stages occur leading up to a feeble 

 development over the greater part of the body; 

 the under-surface of the wing may be naked with 

 the exception of a much reduced single row of 

 under coverts, but intermediate stages occur cul- 

 minating in three rows of coverts ; many degrees 

 in reduction of the upper coverts are also encoun- 

 tered ; the second phalanx of the third finger varies 

 from a free distinct bone to a triangular vestige 

 fused to the end of the first phalanx. On the 

 little toe the claw varies from a stage where it is 

 well developed to one where it is altogether 

 absent, and the scutellation of the big toe may 



