OF SEA- ANEMONES. 93 



represented in Plate II., because it is only to be ob- 

 served by a mucli bigher microscopic power tlian 

 will serve to sbow tbe spikes tbemselves with suffi- 

 cient clearness. 



So far of genera. Our next question really claims 

 a priority in point of time and importance, but, for 

 obvious reasons it comes " last, though not least." 



What are the characteristic marks which warrant 

 us in defining a species ? 



I know of few questions so difficult to answer as 

 this — 



" What is a Species ?" 



" By species," says Professor Balfour, " are meant 

 so many individuals as are presumed to have been 

 formed at the creation of the world, and to have 

 been perpetuated ever since." 



These, when temporarily influenced in form, size 

 or colour by climate, position or nourishment, become 

 varieties. 



*' A variety has a constant tendency to revert to 

 the original type." A "permanent" variety or race 

 may be induced by constant artificial stimulus ; it 

 has still a tendency, though m a lesser degree, to 

 return to the type. 



Now I have collected (see Chapter V.) four or 

 more individual sea-anemones which diff'er entirely 

 from one another in colour, and in this exclusively : 



