METABOLISM. 67 



formation of living protoplasm out of these non-living pro- 

 teids. A cardinal fact is that proteids admit of multi- 

 tudinous transformations; and it seems not improbable that 

 in protoplasm various isomeric proteids are mingled. If so, 

 we must conclude that protoplasm admits of almost infinite 

 variations in nature. Of course pari passu with this dual 

 process — augmentation of protoplasm and accompanying 

 production of carbo-hydrates — there goes extension of plant- 

 structure and plant-life. 



To these essential metabolic processes have to be added 

 certain ancillary and non-essential ones, ending in the forma- 

 tion of colouring matters, odours, essential oils, acrid secre- 

 tions, bitter compounds and poisons : some serving to attract 

 animals and others to repel them. Sundry of these appear 

 to be excretions — useless matters cast out, and are doubtless 

 katabolic. 



The relation of these facts here sketched in rude outline 

 to the doctrine of Evolution at large should be observed. 

 Already we have seen how (§ 8a), in the course of terrestrial 

 evolution, there has been an increasingly heterogeneous as- 

 semblage of increasing heterogeneous compounds, preparing 

 the way for organic life. And here we may see that during 

 the development of plant-life from its lowest algoid and fun- 

 goid forms up to those forms which constitute the chief 

 vegetal world, there has been an increasing number of com- 

 plex organic compounds formed; displayed at once in the 

 diversity of them contained in the same plant and in the 

 still greater diversity displayed in the vast aggregate of 

 species, genera, orders, and classes of plants. 



§ 23cZ. On passing to the metabolism characterizing ani- 

 mal life, which, as already indicated, is in the main a process 

 of decomposition undoing the process of composition charac- 

 terizing vegetal life, we may fitly note at the outset that it 

 must have wide limits of variation, alike in different classes 

 of animals and even in the same animal. 



