310 GENERAL PHYSIOLOGY 



distinction from lifeless substance; while Preyer extends the 

 conception much farther, even to incandescent mixtures, which 

 have not the slightest similarity to present living substance except 

 that they are in energetic motion. If this wide extension of the 

 conception of life be accepted, no objection can be raised against 

 the other consequences of Preyer's theory. It is, however, question- 

 able whether it is judicious or allowable to carry the vital con- 

 ception so far. 



The conception of living substance, as scientifically established 

 at the present time, has arisen from an exact comparison of 

 existing living organisms and existing inorganic bodies. As has 

 been seen, 1 there is but one absolute difference between these two, 

 and this consists in the metabolism of proteids. No inorganic 

 body possesses proteid. On the other hand, proteid is not wanting 

 in any organism ; and that which constitutes the life of an organism, 

 wherein the living differs from the dead organism, is the meta- 

 bolism of the proteid. This difference between living organisms 

 and dead inorganic bodies, although not fundamental or elementary, 

 is, nevertheless, profound and affords the sole means of sharply 

 characterising living substance. If it be nullified by terming 

 " living " bodies that cannot contain proteid, such as the incan- 

 descent masses of the once fiery globe, the advantage afforded by 

 a sharp definition is wholly lost, and the conception of living 

 substance is dissipated. 



But here, from the standpoint of Preyer's theory, the question 

 may be raised : If the living substance of to-day is derived in 

 uninterrupted descent from molten mixtures, where is the limit- 

 beyond which the substance may be termed living ? This ques- 

 tion assumes a postulate that is wholly unsupported, viz., that 

 there was a gradual, uninterrupted transition between the molten 

 mixtures and the proteids. Hitherto we have laid great stress 

 upon the idea that no fundamental difference exists between 

 lifeless substances and organisms ; but it cannot be proved that 

 an uninterrupted transition between molten substances and 

 organisms existed. It is known, moreover, that when two chem- 

 ical compounds act upon one another, the resulting substances are 

 not necessarily joined with the original substances by transition- 

 stages, however different they may be from them. Regarding the 

 relations that may have prevailed upon the earth's surface when 

 water was precipitated as liquid, we cannot form even an approxi- 

 mate idea. The idea that living proteid originated without 

 transition by the action of bodies wholly different from it chemi- 

 cally, when the proper conditions existed, would be at least as 

 probable as the idea of a gradual descent associated with uninter- 

 rupted transitions. 



Further, Preyer implies that the incandescent masses to which 



1 Cf. p. 136. 



