154 THE VICTORIAN NATURALIST. 



molecules, and possibly protoplasm may have many hundred 

 times as many. 



Secondly, living matter holds a very remarkable relation to all 

 the rest of the known universe. It takes up dead materials from 

 that dead universe and converts them into its living self, while at 

 the same time portions of its living self are being continually 

 separated as dead matter, and returned to the dead universe. 

 These latter changes are to a very large extent processes of 

 oxidation, the parts thrown off being parts of the living proto- 

 plasm which have united with the dead O, and are separated 

 chiefly as dead CO,. Thus life is a continual series of changes, 

 the arrest of which means the death of the whole, except in the 

 cases of spores and seeds which have become specially resistant 

 to external influences. In them life may remain dormant or 

 resting for almost idefinite periods provided it is exposed to no 

 conditions which are altogether incompatible with its existence — 

 such as complete drying. What these conditions are will be 

 mentioned later on. Meanwhile, we may notice that the only 

 proof we have of the fact that in such cases life is present, 

 although resting, is the second fact that, under favourable con- 

 ditions, its active manifestations will be resumed. Otherwise, 

 we cannot tell whether life is present or not. 



Now while all living matter is thus being continually broken-up- 

 and changed by the union of the dead oxygen with some of the 

 molecules of the living protoplasm, and the separation of these 

 molecules as dead CO^ — -changes which we comprehend in 

 the case of the higher animals under the name of respiration — 

 provision is also made for its continual renewal. While respiration 

 is the process by which protoplasm is being broken up, digestion 

 and assimilation are the process by which it is being continually 

 renewed. Dead materials are taken up as food, and are in- 

 corporated as part of the living whole. If this process goes on 

 faster than the former, the body grows. If slight but permanent 

 changes take place at the same time in the protoplasm, the 

 process which is called differentiation or development ensues. 



Thirdly, living matter undergoes a series of changes which have 

 a tendency to follow one another in regular succession. Periods 

 of activity alternate with periods of rest. These are seen in the 

 higher animals, which, after all, are aggregates of cells, in sleep and 

 waking, in the tired muscle and the weary brain, in the con- 

 traction and relaxation, the systole and diastole, of the heart ; 

 in the bare boughs of winter and the verdure of smiling spring. 

 But the most remarkable of all these periodical changes is that 

 by which a portion of the living protoplasm becomes detached 

 and starts off on a new and separate existence on its own account. 

 For the most part it inherits all the peculiarities of that from 

 which it has sprung ; in particular, it exhibits these three great 



