528 MR. J. S. HUXLEY ON THE 
~ Tf we want a tabular statement, we can draw up something 
like the following * :—. 
(A) Characters different in the two sexes. 
(Sexual characters). 
(1) Primary. Of the gametes and gonads. 
(2) Accessory. Concerned with the union of the gametes. 
[Copulatory organs, pairing attitudes, 
sexual desire. | 
(3) Secondary. All others. 
(a) Developed through Natural Selection. 
(Huia beak; mamme; marsupium ; 
incubation by @ alone in birds, &e. } 
(b) Capable of being developed through Sexual > Epigamic. 
Selection. | (Courtship characters, i.e. all 
[Horns of deer; tail of Peacock, &e. | characters concerned with 
(B) Characters similar in the two sexes. r the selations of the sexes, 
(1) Capable of being developed through Mutual | excepting those connected 
Selection. immediately with coition.) 
[Grebes’ conrtship and crest ; Herring- 
Gull’s courtship, &c. } 
(2) All other characters. 
It might perhaps be better, as has been suggested to me, to 
restrict the term Secondary Sexual to 3b, and employ Sex-limited 
where I have employed secondary sexual. For one thing, how- 
ever, this would conflict with Darwinian use; also, I am at 
present more concerned to show the necessity for new thinking 
than for new terminology, which will be more suitable in a more 
general and definitive paper. 
IT will conclude by hoping that anyone who has the oppor- 
tunity will observe the habits of the Crested Grebe during the 
time of pairing-up in early spring; the full courtship of the 
Dabchick would also be of very great interest. In the near 
future, I hope to publish a more general paper upon Mutual 
Selection, so that any notes sent to me on this subject will be 
gratefully received and acknowledged. 
For some further discussion, I refer the reader to the Postscript 
(p. 559). 
PART II. 
7. Locatiry, Mrtuops, Erc. 
Through the kindness of the Hon. Walter Rothschild, I was 
given a permit to watch, and a punt to watch from, at the Tring 
Reservoirs. These consist of four large sheets of water in the 
eastern corner of Hertfordshire. Two have dense beds of reeds 
* Simple sexual desire, if we adopt this scheme, we may call an accessory sexual 
emotion. Sexual excitement includes this, and all the epigamic sexual emotions as 
well. Pairing attitudes are accessory sexual attitudes. Let it, by the way, never 
be forgotten that emotions and attitudes are just as much characters as are colours 
or structures. 
