SHELD-DRAKE 837 
1864, pls. 18, 19) of the Australian and New Zealand species, 7’. 
tadornoides + and T. variegata, 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 (dnim. and Pl. under 
Domestic. i. pp. 197-200, 1. 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 (cf. 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 stronger. 
Why it is so we cannot say, yet experiment proves that the 
most widely-different 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 parents. This mysterious 
agency is known as the principle 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 
differentiated 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- Prectroprervs. (After Swainson.) 
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. 
axgyptiaca, and an allied species C. jubata, from South America. 
As shewn by their tracheal characters, the genus Plectropterus, 
composed of the Spur-winged Geese of Africa, and perhaps 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 Anatide. 
1 By inadvertence this species was assigned (p. 600) to New Zealand. 
2 It is further worthy of remark that the young of 7. casarca when first 
hatched closely resemble those of 7. variegata, and when the latter assume their 
first plumage they resemlle their father more than their mother (Proc. Zool. 
Soc. 1866, p. 150). 
