60 EXTERNAL CONDITIONS OF VITAL ACTIVITY. 



of its different actions amongst separate organs, the more close, there- 

 fore, is their mutual dependence, and the more readily, in consequence, 

 are they all brought to a close by the interruption of any one. But 

 there is no doubt, that the actions of even the individual parts of the 

 higher organisms require for their excitement a greater supply of these 

 powers, than the similar actions of the corresponding parts in the lower : 

 whilst if these forces be exerted upon the lower with the intensity that 

 is required for the higher, they destroy the vital properties of the tissues 

 altogether, by the excess of their action. This distinction is most ob- 

 vious in regard to the relative influence of Heat, upon warm-blooded 

 and cold-blooded animals ; of which examples will be given hereafter. 



77. It may also be observed of the influence of these, as of that of 

 other forces whose agency is less general, that it is rather relative than 

 absolute ; being frequently dependent upon the degree of change, rather 

 than upon the measure of the actual amount. This constitutes a marked 

 difference between the influence of these forces on mere chemical com- 

 pounds, and their operation on bodies endowed with vitality. In the 

 former case their action is always uniform ; thus the same amount of 

 heat, the same exposure to light, the same charge of electricity, would 

 be required to produce a given Chemical effect, how often soever the 

 action might be repeated. But this is not the case with living bodies ; 

 since an increase or diminution in the intensity of Heat, which, if made 

 suddenly, would be scarcely compatible with the continuance of Life, 

 may be so brought about, as to produce no marked change in its phe- 

 nomena, the organism possessing a certain power of adapting itself to 

 conditions which are habitual to it, and thus allowing great changes in 

 these conditions to be gradually effected, without any serious disturb- 

 ance. Thus of two individuals of the same species, one may become 

 torpid at a temperature of 60, because it has been accustomed to a 

 temperature of 70 ; whilst another, habituated to a temperature of 60, 

 would require to be cooled down to 50, in order to induce torpidity ; 

 the influence of temperature upon the vital condition being proportioned, 

 more to the variation from the usual standard, than to the actual degree 

 of heat or cold in operation. Yet the first of these individuals might 

 be gradually habituated to live in the same temperature with the second ; 

 and to require the same amount of further depression for the induction 

 of torpidity. (See 132.) 



78. It is a very curious fact, that, whilst the lower classes of living 

 beings are more capable than the higher of bearing the deprivation of 

 these Vital stimuli, they are at the same time more liable to alterations 

 in their own structure and development, in consequence of variations 

 in the degree of their agency, or from other causes external to them- 

 selves. Thus the forms of the lower tribes of Plants and Animals are 

 liable to be greatly affected by the conditions under which they grow ; 

 and these especially modify their degree of development. It seems as 

 if the formative power were less vigorous in the lower, than in the 

 higher classes; so that the mode in which it manifests itself in the 

 former is more dependent upon external influences ; whilst in the latter 

 it either predominates over them, causing the regular actions to be per- 

 formed, or gives way altogether. The same principle applies to the 



