88 EVOLUTION AND ETHICS n 



successive intervals, it develops buds which grow into 

 strawberry plants ; and these become independent by 

 the death of the parts of the sucker which connect 

 them. The rest of the sucker, however, may go on 

 living and growing indefinitely, and, circumstances 

 remaining favourable, there is no obvious reason why 

 it should ever die. The living substance B, in a 

 manner, answers to the sucker. If we could restore 

 the continuity which was once possessed by the por- 

 tions of B, contained in all the individuals of a direct 

 line of descent, they would form a sucker, or stolon, on 

 which these individuals would be strung, and which 

 would never have wholly died. 



A species remains unchanged so long as the 

 potentiality of development resident in B remains 

 unaltered ; so long, e.g., as the buds of the strawberry 

 sucker tend to become typical strawberry plants. In 

 the case of the progressive evolution of a species, the 

 developmental potentiality of B becomes of a higher 

 and higher order. In retrogressive evolution, the 

 contrary would be the case. The phenomena of 

 atavism seem to show that retrogressive evolution, 

 that is, the return of a species to one or other of its 

 earlier forms, is a possibility to be reckoned with. 

 The simplification of structure, which is so common 

 in the parasitic members of a group, however, does 

 not properly come under this head. The worm-like, 

 limbless Lerncea has no resemblance to any of the 

 stages of development of the many-limbed active 

 animals of the group to which it belongs. 



