CODKTSHIP ACTIVITIES IN THE RED-THROATED DIVEK. 285 
The differentiation of action between the sexes is thus very considerable ; yet 
in spite of this, as I say, either sex adopts either position according to 
circumstances. In the Moorhen, Selous (^02) has described reversed pairing 
as a normal sequence of pairing in the usual position. Mr. E. Howard tells 
me that he has repeatedly had occasion to confirm this himself. In Doves 
and Pigeons it may also occur (Whitman & Riddlej '19 ; Marshall, '22, p. 690). 
It should be remembered that sexual dimorphism to any marked extent is 
a late development in most groups. Primary and accessory organs are, 
naturally, different, but general coloration and body-form is usually similar in 
both sexes in primitive animals — e.g. Bchinoderms, many primitive insects ; 
Crustacea and other Arthropoda, Cyclostomes and most fish. In any case, 
recent work has made it abundantly clear that the genes for the secondary 
sexual characters of both sexes are normally carried in all members of the 
species, and that either the sex-chromosome mechanism ensures two quite 
different types of cellular metabolism, one permitting the development of 
male, the other of female characters, as seems to be the case in insects, or 
else that special hormones are developed in the gonads which exert specific 
growth-promoting effects upon some sexual characters, inhibiting effects 
upon others, as in vertebrates (summaries in Marshall, ^22, ch. 15 ; 
Goldschmidt, '20 ; Huxley, '22 a). 
If, therefore, the divergence which I have referred to between the sexually 
dimorphic and mutualist types of birds originated early, as it seems to have 
done (for the distinction characterizes whole groups of the class), we may 
suppose that the primitive bird species on which it acted were probably 
similar in appearance, without well-marked courtship colours or structures, 
whether in one or in both sexes, and with a moderate dimorphism of instincts! 
Later development has either accentuated the difference of instincts, as in the 
Sylviidse, or the divergence of instincts and of appearance, as in most 
Grallinaceous birds ; or has allowed bright colours to develop, apparently for 
use in courtship, but has not confined them to one sex, although it has not 
closely approximated the instincts of the two sexes (as in the Penguins, 
Levick, loc. cit.), or finally has encouraged sexual adornments similar in the 
two sexes, together with markedly mutual courtship, and has approximated 
instincts to a high degree — as in the Grebes. 
So far as observation can be a guide in these matters, it appears that 
although sex-dimorphism in plumage may depend upon different genetic and 
physiological factors from that in instincts and from that in size, yet as a 
matter of evolutionary fact, the first two, and probably the third also, have 
gone hand in hand. ^ 
In other words, the divergence between dimorphic and mutualist species is 
primarily dependent on whether the gonadial hormones remain as similar as 
is compatible with primary sex-differentiation, or whether they become con- 
siderably different in the two sexes. If they adopt this latter alternative. 
