INFLUENCE OF HEAT ON ANIMALS. 89 



said of their nutritive and developmental operations. Thus the rate at 

 which regeneration of lost parts, like that of the ordinary process of 

 budding, takes place in the common Hydra (Fresh-water Polype), is in 

 close accordance with the temperature in which it lives ; and in like 

 manner, the healing of wounds in Frogs takes place more rapidly in 

 summer than in winter. In many of the higher animals, indeed, 4t 

 appears that the complete regeneration of parts requires a higher tem- 

 perature than is necessary to sustain the ordinary vital activity. Thus 

 it has been found that the common Triton (water-newt) can reproduce a 

 limb that has been cut off, if it be kept at a temperature of from 58 to 

 75; but cannot do so if a less amount of heat be afforded to it. And 

 in like manner, the snail can regenerate its head, if it be kept in a warm 

 atmosphere, but not at a low temperature. Now it has been justly re- 

 marked by Mr. Paget, that the process of development seems to require 

 a higher amount of vital force than simple growth ; and we see that the 

 relation already pointed out between Heat and Vital force, here holds 

 good in such a marked degree, as to afford a strong confirmation of the 

 idea of their mutual relationship. 



130. It is quite conformable to the same principle, that we should 

 find Cold-blooded animals able to sustain the deprivation of food during 

 a much longer period, at cold temperatures, than at warm. The case 

 is precisely the reverse, however, in regard to most Warm-blooded ani- 

 mals ; since in them a due supply of food is a condition absolutely ne- 

 cessary (as we have already seen) for the maintenance of that amount 

 of bodily heat, whose loss is fatal to them ; and exposure to a low tem- 

 perature will of course more speedily bring about that crisis. Hence it 

 is that Cold and Starvation combined are so destructive to life. But in 

 this respect also, the hybernating Mammals correspond with the cold- 

 blooded classes ; their power of abstinence being inversely as the tem- 

 perature of their bodies. 



131. We have seen that the animals termed cold-blooded are greatly 

 influenced as to the temperature of their bodies, by the temperature of 

 the surrounding medium ; although many of them are endowed with the 

 power of keeping themselves a certain number of degrees above it. Now 

 the consequence of this is, that all of them which are subject to any 

 considerable and prolonged amount of cold, pass into a state of more or 

 less complete inactivity during its continuance ; which state bears a close 

 correspondence with the hybernation of certain Mammalia. Among the 

 Reptiles of cold and temperate countries, this torpid state uniformly 

 occupies a considerable part of the year ; as it does also with Insects, 

 terrestrial Molluscs, and other Invertebrated animals, which are subject 

 to the influence of the cold. On the other hand, Fishes, Crustacea, and 

 other marine animals, do not usually appear to pass into a state of tor- 

 pidity ; the temperature of the medium they inhabit never undergoing 

 nearly so great a degree of depression, as that of the atmosphere. The 

 amount of change necessary to produce this effect, or on the other hand 

 to call the animals from a state of torpidity to one of active energy, 

 differs for different species ; and there is probably a considerable diffe- 

 rence even among individuals of the same species, according to the tem- 

 perature under which they habitually live. Thus one animal may remain 



