REMARKS OX MR DAR^VIN's THEORY. 
183 
oxygen, less carbonic acid — after a plant has grown in it than before. 
True, they give out carbonic acid at night, but not so much as they 
take in. All the plant (except water) is so much gained from this 
carbonic acid. Hence, the air is purified by plants. 
Now coal being of vegetable origin, it is calculated that for every 
pound of coal, all this carbon, and at least two pounds of icater have 
disappeared from the atmosphere. And if we consider the milhons 
upon millions of tons, fixed in solid black masses in the crust of the 
earth, we must see that we are living in an atmosphere far purer, and 
more fit for the respiration of the higher animals, than it could have 
been without the aid of coal. 
It may have been, as the sagacious De la Beche observed, that 
this enormous supply of carbonic acid was due to the ejections from 
many volcanic mouths, which we know breathed forth their fiery 
exhalations in coal times. It is also true, as Sir C. Lyell has said, 
that these gases so readily mix with the atmosphere, that little appre- 
ciable difference would be made by any quantity of volcanic action. 
But look at the subject in any light we may, there was the carbonic 
acid in the air, and there it now is, for our benefit, in the earth. 
This rank vegetable produce, then, of quick growth and soft tissue 
— constantly wet, fermenting as soon as covered up — its heat kept in 
by a blanket of wet sand or clay, with pressure for ages, gives us all 
the conditions necessary for the production of lignite, brown coal, 
jet, and pit- coal ; and when volcanic heat had driven away its gaseous 
parts, and left the carbon pure — even anthracite. 
As this month's communication has extended to an unreasonable 
length, I will not now enter into the question of the difierent qualities 
of coal, or its uses, but defer what little I have to say on those 
subjects till next month. 
SOME REMARKS ON MR. DARWIN'S THEORY. 
By Frederick Wollastox Hutton, F.G.S. 
(Continued from ])age 136). 
But there are other causes that have tended to modify animals ; 
such as habit, use or disuse of any particular organ, food, climate, &c., 
and these together Avith the fact that a variation which appears in the 
parent, at any period of its existence, tends to re-appear in the off- 
spring at the same period, vdW. enable us to account for the metamor- 
phoses of insects, the differences of colour in the young and the 
adult, the horns of sheep and cattle, &c. If to these we add that of 
" sexual selection,"* we can see why sexes differ in organs and pro- 
* Sexual Selection may be defined as the preference shown by an individual of 
one sex for an individual of the other from superior beauty of coloui-, shape, voice, &c. 
