THE VITAL AND PHYSICAL FORCES. 
749 
suffices for their growth in the larva state, being retarded or even prevented by the 
want of a due amount of these agencies (see p. 753); and it has been also shown by 
Mr. Higginbottom that the development of new limbs in the Triton, to replace those 
which have been lost, cannot take place at a lower temperature than about 60°, although 
the processes of growth go on under a much less degree of heat*. The general proposi- 
tions enunciated by Prof. Milne-Edwards -f', in regard to the geographical distribution 
of the Crustacea, indicate the existence of this relation in the most decided manner. 
They are, briefly, as follows ; — I. The varieties of form and organization (which may 
be regarded as so many varied manifestations of the organizing force) increase as we 
pass from the Polar Seas towards the equator, the number of species tlius augmenting 
greatly as we go southwards. II. The differences of form and organization are not 
only more numerous and more characteristic in the warm than in the cold regions of 
the globe ; they are also more important. III. Not only are those Crustacea which 
are most elevated in the scale deficient in the polar regions, but their relative number 
decreases rapidly as we pass from the equator towards the pole. IV. The average 
size of the Crustacea of tropical regions is considerably greater than that of the tribes 
inhabiting temperate or frigid climes. V. It is where the temperature is most ele- 
vated, that the peculiarities of structure which characterize the several groups are 
most strongly manifested. And VI., there is a remarkable coincidence between the 
temperature of different regions, and the prevalence of certain forms of Crustacea. — It 
is interesting to observe, that the want of a high temperature is no obstacle to the 
growth and multiplication of individuals of a comparatively small size and low grade 
of organization ; the Arctic and Antarctic seas being as numerously peopled with 
such, as the tropical ocean is with higher forms. But the preceding statements point 
to a direet and definite relation between Heat and the Organizing force, as mani- 
fested in this group of animals. A comparison of the facts relating to the geographical 
distribution of other classes of cold-blooded animals would probably justify the same 
conclusions. There can be no doubt of their general applicability to the Vegetable 
kingdom. 
5. The influence of temperature upon the general vital activity of cold-blooded 
animals is no less remarkable. The facts determined by the experiments of Dr. W. F. 
Edwards:|: lead to this general conclusion ; — that the rate of life of Batrachia and 
Fishes, of which the activity of their respiratory process is the exponent, varies 
directly (within certain limits) as the temperature of the surrounding medium ; so 
that the duration of life, when these animals are deprived of air, either partially or 
completely, or are placed in any other circumstances unfavourable to its sustenance, 
varies inversely with the external temperature. Thus when frogs were confined in a 
limited quantity of water, and were not allowed to come to the surface to breathe, it 
* Proceedings of the Royal Society, March 18, 1847. 
t Histoire des Crustaces, tom. iii. p. 555 et seq. 
J On the Influence of Physical Agents on Life, passim 
