1921 .] 
Astronomical Notes. 
139 
to be streaming along the spiral arms either inwards or outwards. The 
coiling results from the persistence of angular momentum. The outer 
stars lag behind ; the inner ones forge ahead. Thus the development of 
a spiral form is inevitable. To give a homely illustration, if an extended 
line of soldiers were to wheel about a point in the centre of their line, each 
soldier moving in his own circle concentric with that point with a velocity 
common to all, the formation would soon become a double spiral. 
The question may arise why all the distant nebulae are exclusively of 
this type. Is every nebula the result of an encounter ? The answer is 
that we see those only that are rendered luminous by such interpenetration. 
We never see the ones that are going into action, because visibility is one 
of the effects of the conflict. Nor can we ever hope to see the outburst 
of a new spiral as we see the outburst of a nova, for the simple reason that 
while the encounter of two sunlike stars takes about three-quarters of an 
hour, that of two cosmic systems takes hundreds or thousands of millions 
of years. 
After untold ages of evolution the greater proportion of even the 
invisible nebulae must now be spiral, for whatever the original nature of 
cosmic systems may be the interpenetration of one with another impresses 
on the united mass the double-spiral form. The visible nebulae owe their 
wonderful diversity to the peculiarities of the encountering systems. But 
through all their endless variety one characteristic persists and is recognized 
universally. 
The hall-mark of its birth, the double-spiral form, proclaims eternally 
that the nebula sprang from a celestial encounter involving not single stars 
but vast cosmic systems. 
NATURAL-HISTORY NOTES. 
The Short-tailed Bat ( Mystacops tuberculatus Gray), by J. G. Myers, 
F.E.S. 
In January last Mr. P. M. Brooker caught a specimen of this rare bat 
in the bush on the western slopes of the Tararua Range. It entered the 
hut apparently attracted by candle-light. Although the party was camped 
in the neighbourhood for some weeks this was the only example seen. 
Both of the native bats are now rare ; but, although the long-tailed 
species (Ghalinolobus mono Gray) has at times occurred in considerable 
numbers, the species under consideration has been extremely rare as far 
back as our records go. Hutton and Drummond gave the latest record of 
its occurrence as 1871, and venture the opinion that it might possibly 
be then (1905) extinct. Of the two stuffed specimens in the Dominion 
Museum the most recent was acquired some forty years ago. The reap¬ 
pearance, therefore, of the species after so considerable a lapse of time is 
of great biological interest ; and this on account not only of its rarity and 
of the interest attaching to the single species of a precinctive genus, but 
also of certain points in its structure which render it unique. Dobson 
{Catalogue of Chiroptera in the British Museum) pointed out nearly fifty 
years ago the probability that the very thorough and striking manner in 
which the thinner membrane of the wings is protected when at rest, and 
the denticles at the bases of the extremely sharp pollical and pedal claws, 
are adaptations to a more or less arboreal life. He suggested that a 
study of its bionomics should be undertaken to ascertain in the light of this 
