274 
When the films of decomposed glass are circular spherical seg- 
ments and colourless, the black cross and its accompanying tints are 
finely displayed, as in the system of rings seen along the axis of 
uniaxal crystals. When the films have the colour of thin plates, 
and 'are deeply spherical segments, the tints of the rings which ac- 
company the black cross are singularly modified. 
2. On Mr Darwin’s Theory of the Origin of Species. 
By Andrew Murray. 
The position taken by Mr Darwin is, that all species have arisen 
by the natural process of ordinary generation. That the differences 
which we now see in them have arisen from slight variations in in- 
dividuals having from time to time occurred, which have been per- 
petuated by. inheritance, by successive stages and slow degrees, 
through unlimited spaces of time. Some of these slight variations 
he considers to originate in causes beyond our power of explanation, 
and which, although not the work of chance, we may call chance, for 
want of a better appellation- — others to arise from habit, or from 
the excessive use or disuse of certain organs ; but that when such a 
variation has once appeared, it is preserved by hereditary descent 
through a principle which he calls “ natural selection,” and which 
he deduces as a corollary from the struggle for existence which we 
see constantly going on around us. “ As many more individuals,” 
says he, “ of each species are born than can possibly survive ; and as, 
consequently, there is a frequently recurring struggle for existence, it 
follows that any being, if it vary, however slightly in any manner 
profitable to itself, under the complex and sometimes varying con- 
ditions of life, will have a better chance of surviving, and thus 
be naturally selected . From the strong principle of inheritance 
any selected variety will tend to propagate its new and modified form” 
(p. 5). Mr Darwin by no means shuns pushing his theory to its legiti- 
mate conclusion. In arguing as to the acquisition of new habits by 
some of his supposed transitional animals, he says, “ In North 
America the black bear was seen by Hearne swimming for hours 
with widely-open mouth, thus catching, like a whale, insects in the 
water. Even in so extreme a case as this, if the supply of insects 
were constant, and if better adapted competitors did not already 
exist in the country, I can see no difficulty in a race of bears 
