COURTSHIP OF THE GREAT CRESTED GREBE, 505 
successful: here the active bird was in both cases the smaller of 
the pair. 
Now, if we could be sure that the 1901 pair was the same that 
was there in 1900, all would be well: but we cannot be sure. 
There was a marked difference in the pairing-behaviour of the 
1900 and 1901 pairs—a difference that cannot be referred back 
to the fact that in 1900 the birds were building a true nest and 
were incubating, while in 1901 they had only got to the length 
of building a pairing-platform. In 1900 the smaller bird (that 
we have so far presumed to be the hen) was more forward in 
invitation, while the active pairing-position was adopted by the 
larger bird alone; im 1901 the case was exactly reversed. It 
would be, in my opinion, more remarkable that such a change 
of character should take place in two birds in the space of 
one year than that the same water should be occupied by two 
different pairs—albeit but a single one—in two successive years. 
Mr. Selous, however, writes to me that for various reasons 
(e. g., the site of the nest, etc.) he is practically convinced that 
the birds were the same in both years. However, whether the 
pair was the same pair or not, in both years there was a marked 
difference in size both of body and crest between the two birds 
of the pair, and, if all the books are not wrong, this should be 
quite enough to distinguish the sexes. Sometimes, it is true, 
the two birds of a pair are almost exactly alike; but nowhere 
do I find it stated that the hen is ever larger or has a better 
crest than the cock. It is the part of the professional orni- 
thologist to find out if this is ever so; till then, we must be 
content to say that it is extremely probable that either cock or 
hen can play the “active” part in copulation—what we should 
usually call the male part. This can be more easily imagined 
in birds than in almost any other animals in which copulation 
takes place, but even in a bird is remarkable enough. Definite 
attitudes of the two participating organisms have been evolved to 
facilitate the passage of genital products in a definite direction : 
and here, hey presto! although the genital products continue to 
pass in the same direction, yet the attitudes, developed only in 
relation with and accessory to this direction, are at will reversed. 
This facultative reversal of pairing-position would certainly be 
remarkable; but even for the moment supposing that it does 
not occur in our Grebe, it would merely appear as the as yet 
unattained end of a process of sex-equalization which in this 
species has already run a considerable course. This process 
consists in a gradual transference of all the secondary sexual 
characters of the male to the female, and vice versa. In its 
general aspect it will be discussed later; here it will be sufficient 
to consider it in relation to the pairing actions alone. 
Let us see what is without doubt common to both sexes in the 
Crested Grebe to-day. First of all, we find that either cock or 
hen may lead the way towards the pairing-platform. Secondly, 
either cock or hen may assume the passive pairing-positien (the 
