RESISTANCE TO HEAT AND COLD. 15 



destruction of its fundamental substance at that heat, and 

 how far death is brought about by the coagulation of merely 

 accessory compounds. 



It may be safely said of all those living things which are 

 large enough to enable us to trust the evidence of micro- 

 scopes, 1 that they are heterogeneous optically, and that their 

 different parts, and especially the surface layer, as contrasted 

 with the interior, differ physically and chemically ; while, in 

 most living things, mere heterogeneity is exchanged for a 

 definite structure, whereby the body is distinguished into 

 visibly diverse parts, which possess different powers or func- 

 tions. Living things which present this visible structure are 

 said to be organized / and so widely does organization obtain 

 among living beings, that organized and living are not unfre- 

 quently used as if they were terms of coextensive applicabil- 

 ity. This, however, is not exactly accurate, if it be thereby 

 implied that all living things have a visible organization, as 

 there are numerous forms of living matter of which it cannot 

 properly be said that they possess either a definite visible 

 structure or permanently specialized organs : though doubt- 

 less the simplest particle of living matter must possess a 

 highly-complex molecular structure, which is far beyond the 

 reach of vision. 



The broad distinctions which, as a matter of fact, exist 

 between every known form of living substance and every other 

 component of the material world, justify the separation of 

 the biological sciences from all others. But it must not be 

 supposed that the differences between living and not-living 

 matter are such as to bear out the assumption that the forces 

 at work in the one are different from those which are to be 

 met with in the other. Considered apart from the phenomena 

 of consciousness, the phenomena of life are all dependent 

 upon the working of the same physical and chemical forces 

 as those which are active in the rest of the world. It may 

 be convenient to use the terms " vitality " and " vital force " to 

 denote the causes of certain great groups of natural opera- 



1 In considering the question of the complication of molecular structure 

 which even the smallest and simplest of living beings may possess, it is well 

 to recollect that an organic particle tooto of an inch in diameter, in which our 

 best microscopes may "be incompetent to reveal the slightest differentiation of 

 parts, may be made up of 1,000,000 particles Tpmpoo of an inch in diameter, 

 while the molecules of matter are probably much less than xpalpoo of an inch in 

 diameter. Hence in such a body there isample scope for any amount of com- 

 plexity of molecular structure. 



