ADAPTATION AND PROGRESS 109 



stone is set upon stone. If, on the contrary, the unity 

 of life is to be found solely in the impetus that pushes 

 it along the road of time, the harmony is not in front, 

 but behind. The unity is derived from a vis a tergo : 

 it is given at the start as an impulsion, not placed 

 at the end as an attraction. In communicating itself, 

 the impetus splits up more and more. Life, in pro 

 portion to its progress, is scattered in manifestations 

 which undoubtedly owe to their common origin the 

 fact that they are complementary to each other in 

 certain aspects, but which are none the less mutually 

 incompatible and antagonistic. So the discord between 

 species will go on increasing. Indeed, we have as yet 

 only indicated the essential cause of it. We have 

 supposed, for the sake of simplicity, that each species 

 received the impulsion in order to pass it on to others, 

 and that, in every direction in which life evolves, the 

 propagation is in a straight line. But, as a matter of 

 fact, there are species which are arrested ; there are 

 some that retrogress. Evolution is not only a move 

 ment forward ; in many cases we observe a marking 

 time, and still more often a deviation or turning back. 

 It must be so, as we shall show further on, and the 

 same causes that divide the evolution movement often 

 cause life to be diverted from itself, hypnotised by the 

 form it has just brought forth. Thence results an 

 increasing disorder. No doubt there is progress, if 

 progress mean a continual advance in the general 

 direction determined by a first impulsion ; but this 

 progress is accomplished only on the two or three 

 great lines of evolution on which forms ever more 

 and more complex, ever more and more high, appear ; 

 between these lines run a crowd of minor paths in 

 which, on the contrary, deviations, arrests, and set-backs 



