LIFE-HISTORY OF XENOPUS LAVIS, DAUD. TOY 
why some animals appear in great numbers in one year and are much less numerous 
in another. Entomologists are familiar with this phenomenon, and it may be worth 
considering whether such fluctuations in numbers are not due to causes of the 
nature indicated. 
It is obvious that changes in the environment affecting the breeding habits might 
lead to rapid divergence through the action of Natural Selection, and the diversity 
in the breeding habits of allied tailless Batrachians has perhaps been established 
through the agency of such induced sterility. 
The male Xenopus begins to assume nuptial characters a couple of days after 
the temperature is raised to 22° C. The dorsal surface of the hand darkens and 
the area covered with nuptial asperities extends along the arm towards the axilla; 
the whole patch blackens from the hand inwards in the course of about two days. 
The shape of the patch has already been figured (Buus, ’01). 
The abdomen of the female becomes very much distended during the winter by 
the enormously enlarged ovaries, so much that the lungs are displaced upwards 
and raise the dorsal body-wall on either side of the vertebral column into two 
great projecting longitudinal humps. The three flaps of skin surrounding the cloacal 
aperture are flaccid until the spring, when they become swollen and turgid and 
more highly vascularised. I was unable to detect any change in the epidermis 
of the breeding female until last year (1903), when the back of the hands 
became darker at the same time as the nuptial asperities appeared in the male. 
Special attention was paid to this pomt in the seasons before, and it is certain 
that nothing of the kind occurred then, so that it appears that a secondary sexual 
character of the male is making its appearance with age in the female (see BouLENGER, 
97, p. 72, for similar cases). 
During the first week of the newly established spring conditions the males become 
vocal. They have been silent throughout the winter, and their first attempts are 
intermittent and low in tone. Their voice strengthens from day to day, and at 
night-fall, especially if fresh water has been added, becomes a loud and continuous 
metallic rattle, kept up for hours with hardly a break. The noise made by a 
single frog is loud enough to be heard at a distance of 100 yards or more in 
the open.* It resembles the voice of Hyla arborea more than that of any European 
frog, but has two alternating notes extremely like those made in winding up an 
old grandfather’s clock with a crank handle. By rubbing the corrugated handle 
of a pair of large forceps backwards and forwards against the rounded edge of 
an empty tin tobacco box, I have imitated the sound so exactly that the frogs 
have responded. The croak is produced under water, and although air is no doubt 
passed to and fro between the lungs and the buccal cavity, there is no movement 
of the pectoral or gular region visible externally. 
Normally, pairing only occurs at night. The male croaks loudly and incessantly 
* J had not heard its full strength when I made the statement in my former account (BuEs, ‘01, p. 211). 
TRANS. ROY. SOC. EDIN., VOL. XLI. PART III. (NO. 31). 117 
