R^SUMf 335 



We observe (although we must not overestimate the 

 regularity of the processus) that higher forms make 

 their appearance after and at the expense of lower 

 forms, and that this evolutionary process is never re- 

 versible. As this evolution does not coincide with any 

 progressive adaptation it cannot be explained 

 through natural selection nor through the Lamarckian 

 factors. We cannot account for it unless we go back 

 to the origin of life, to the physico-chemical constitu- 

 tion of the cell. In the course of organic life the cell 

 acquires new chemical substances and is submitted to 

 new physical actions. None of those actions ceases 

 without leaving some traces; their effects are, so to 

 speak, stored up, while the chemical constitution of the 

 cell-substance becomes more and more complex. The 

 number of chemical substances it contains increases 

 steadily, permitting a more and more varied histolog- 

 ical differentiation and contributing to the growing 

 complexity of the organism. This of course is a 

 rather general explanation, and the problem will not 

 be solved until we can reallv observe this increase in 

 complexity and understand clearly how a certain chem- 

 ical processus can give rise to a certain histological 

 structure. 



The problem of differentiation inevitably introduces 

 another problem. We know that the plasticity of 

 beings is in inverse ratio to their differentiation and 

 that in every animal group it is the least differentiated, 



