384 SECTIONAL TRANSACTIONS.—D. 
At the beginning of the flight the fins vibrate passively as a result of the 
movements of the tail, which at this time is being actively vibrated below 
the surface of the water. During the remainder of the flight no evidence 
can be obtained of any flapping of the fins by the fish. That no such 
flapping occurs is confirmed by the results of dissections of the fin muscles. 
These dissections show that no muscles are present which could result 
in flapping movements by their contraction. It must be concluded that 
the flight is a glide, and that after the preliminary stage, when the tail is 
vibrated below the surface of the water, the fish obtains no energy from the 
contraction of its muscles. 
From measurements of cinematograph photographs of the fish in flight, 
taken by Mr. E. N. Willmer during a voyage across the Atlantic, it has 
been calculated that the normal flying speed of the fish is never greater than 
25-30 miles an hour. This is too low a speed to enable a fish of the size 
and weight of Exocetus to glide in still air for the distances and times which 
are often observed. It is suggested that disturbance of the air above the 
uneven surface of the water may be the source from which the fish is able 
to obtain energy for a longer glide than would be possible in still air. 
AFTERNOON. 
Prof. F. A. E. Crew.—Colour inheritance in the Budgerigar (2.15). 
Attention is drawn to the value of the budgerigar as a material for the 
experimental study of evolution. All the existing colour varieties have 
arisen out of one wild type within the memory of living man, and it is known 
that the wild type form itself has not been crossed with any other species. 
A mutant gene (b) is responsible for the blue plumage colouration ; 
(y) for the yellow ; (D) in the simplex state turns light green into dark green, 
and in the duplex state into olive ; sky-blue into cobalt and mauve ; yellow 
into dark yellow and yellow-olive. Greywing (gr) is an allele of yellow 
which lightens all pigmentation, especially that of the wing undulations. 
Cinnamon (cn) is a sex-linked recessive replacing the black by brown. 
It is not yet known whether the buttercup colour (yellow without the green 
suffusion) is another allele of yellow or is due to modifiers. White is the 
homozygous blue-yellow compound. Fallow, lutino and albino are new 
characters, the genetics of which is not yet known. 
The budgerigar is of interest also because autosomal colour mosaics 
are common. A series of some two dozen of these ‘ halfsiders ’ is discussed 
and explained by an appeal to the chromosome elimination hypothesis. 
Prof. W. GarstaNc.—The marine biological station at Bermuda (2.45). 
Dr. H. B. Cotr.—The nature and function of disruptive colouration in 
animals (3.15). 
Concealment by means of adaptive colouration is an important factor 
in the lives of different animals. The problem of concealing colouration 
must be approached as a field study : the significance of colour and pattern 
can only be appreciated in reference to the habits of animals in nature, and 
to the habits of potential enemies and prey. The function of concealing 
colouration is deception—the rendering of an animal unrecognisable. 
Theoretically the concealment of an exposed object depends for its success 
upon the creation of certain optical illusions, underlying which there are 
three fundamental principles: (1) the resemblance in colour between an 
