SHELD-DRAKE 



837 



1864, pis. 18, 19) of the Australian and New Zealand species, T. 

 tadornoidcs ^ and T. variegafa, it will at once be seen that the hybrids 

 present an appearance almost midway between the two species last 

 named — species which certainly had nothing to do with their pro- 

 duction. The only explanation of this astounding fact seems to 

 be that afforded by the principle of " reversion," as set forth by 

 Mr. Darwin, and illustrated by him from examples of certain 

 breeds of Doves, domestic Fowls and Ducks (Ahwi. and PI. under 

 Domestic, i. pp. 197-200, ii. p. 40), as well as, in the matter of 

 domestic Fowls, by Mr. Cambridge Phillips {Zool. 1884, p. 331). 

 It is a perfectly fair hypothesis that the existing animals of New 

 Zealand and Australia {rf. GEOGRAPHICAL Distribution, pp. 315- 

 317) retain more of their ancestral character than do those 

 of countries in which we may suppose the struggle for life to 

 have been fiercer and the action of natural selection stronejer. 

 AVhy it is so we cannot say, yet experiment proves that the 

 most widely -difierent breeds of Pigeons and other poultry, when 

 crossed, produce offspring that more resembles the ancestral wild 

 species from which the domesticated forms have sprung than 

 it resembles either of the immediate jj^rents. This mysterious 

 agency is known as the jirinciple of " reversion," and the example 

 just cited proves that the same effect is produced in species as well 

 as in "races," — indicating the essential identity of both, — the 

 only real difference being 



that " species " are more _,„*f— -^ ' --^m^=^^ 1^ 



ditferentiated than are 

 "races," or that the dis- 

 tinction between them, 

 instead of being (as many 

 writers, some of the first 

 repute, have maintained) 

 qualitative, is merely quan- 

 titative, or one of degree.'-^ 



The genus Tadorna seems to be most nearly related to Chenalo- 

 pex, containing the bird so well known as the Egyptian Goose, C. 

 xgyptiaca, and an allied species C. juhata, from South America. 

 As shewn by their tracheal characters, the genus I'ledropterus, 

 composed of the Spur-winged Geese of Africa, and jierhaps the 

 Australian Anseranas and the Indo-African Sarcidiornis, also appear 

 to belong to the same group, which should be referred rather to 

 the Anatine than to the Anserine section of the Anatidx. 



Plectropterds. (After Swaiiison.) 



^ By inadvertence this species was assigned (p. 600) to New Zealand. 



" It is further worthy of remark that the young of T. casarca when first 

 hatched closely resemble those of T. variegata, and when the latter assume their 

 first plumage they resemble their father more than their mother {Proc. Zool. 

 JSoc. 1866, p. 1.50). 



