THE VITAL AND PHYSICAL FORCES. 
753 
real obstacle to the reception of this doctrine ; since in any hypotliesis which assumes 
a common force as operative in the living' kingdoms of nature, it is necessary to 
admit that this force is modified in its action by the properties of the germ, just as 
that the general force of chemical affinity manifests itself differently in the reactions 
of each elementary and composite substance. And just as the chemist seeks to de- 
termine the laws of chemical affinity by observation and experiment, so does the 
philosophic physiologist aim to discover the general plan on which the vital force is 
exerted, in the production of the wonderful series of organized structures which have 
successively presented themselves on this globe. 
In speaking of Heat as the physical agent especially concerned in the development 
of living organisms, and in the maintenance of their activity, the author would by 
no means leave out of view the other physical forces, all of which, if correlated to 
each other, as well as to the vital forces, must be capable of exerting an important 
influence on these processes. He has merely selected Heat, as the one whose opera- 
tion is most extensive and most easily demonstrated ; and every fact wffiich indicates 
that other physical agencies are also in operation, will (of course) only add weight to 
his argument. To the universally-admitted agency of Light, in directly exciting one 
(at least) of the most important processes of vegetable growth, reference has already 
been made. But there is evidence that light has an influence upon certain processes 
of development, which cannot be accounted for by its agency in the fixation of car- 
bon from the atmosphere, and in the production of organic compounds. One of the 
most remarkable examples of this agency is furnished by the experiments of Mirbel 
upon the gemmae of Marchantia polymorpha. He found, after repeated trials, that 
during the development of these little discs, stomata are formed on the side exposed 
to the light, whilst root-fibres grow from the lower surface ; and that it is a matter 
of indifference which side of the disc is at first turned upwards, since each has the 
power of developing stomata, or roots, according to the influence it receives^. The 
experiments of Dr. W. F. Edwards indicate that a decided influence is exerted by 
light upon the metamorphosis of the Batrachia ; since, according to his statements, 
when tadpoles, arrived at nearly their full growth, were secluded from the influence 
of light, but were supplied with aerated water and food, they continued to increase as 
tadpoles (so as to attain an extraordinary size, doubling or even trebling their usual 
full weight) without undergoing any metamorphosis 'f'. The influence of light upon the 
* Nouvelles Annales du Museum, tom. i. 
■f On the Influence of Physical Agents on Life, p. 53. — The results of the recent experiments of Mr. Hio- 
GiNBOTTOM appear to negative those obtained by Dr. EnwAuns, and to show that metamorphosis is only re- 
tarded by privation of light, when accompanied by reduction of temperature. But the remarkable fact above 
quoted from Dr. Edwards’s statements, to which Mr. Higginbottom has recorded nothing parallel, shows 
that there was some difference in the conditions of the two sets of experiments, which should prevent us from 
setting aside those statements, made (as they are) by a most trustworthy observer, until they shall have been 
more fully disproved. 
5 D 
MDCCCL. 
