200 THE BIOLOGY OF THE SEASONS 



like Lingula, appearing in the Cambrian and living on 

 apparently unchanged until to-day, we should never have 

 had any theory of organic evolution to puzzle over. We 

 may regard Lingula, then, as the supreme instance of 

 static racial inertia ; and it does not stand alone. But 

 to judge of the history of animal life from cases like Lingula 

 is, as has been said, like judging the flow of a river by a 

 study of a small, quiet pool on its shore. 



In contrast to the conservative Lingula we may take 

 the common jellyfish (Aurelia aurita) as a good illustration 

 of variability. It is constantly exhibiting variations, that 

 is, inborn changes of germinal origin which result in the 

 organism being different from the norm or average of its 

 species. We know from the records of zoology that it has 

 kept up this sporting for a great many years, though 

 nothing seems to come of it after all. It is normally a 

 tetrapartite animal, but sexpartite, pentapartite, and, more 

 rarely, tripartite forms occur ; and the detailed variations 

 are manifold. Similarly, whether we take pansies or 

 poultry, wild animals or tame, we get the same impression 

 of many a living creature as a Proteus. Whether we 

 examine the vertebrae of sloths or the teeth of apes, the 

 colour of dog-whelks or of guillemots' eggs, the external 

 characters of land-snails or of ruffs, we have the big fact of 

 variability staring us in the face. And whenever we begin 

 to compare specimens with their " specific characters," 

 which are only averages, whenever we take the trouble to 

 measure and weigh, we discover that the fountain of 

 organic change is more copious than even Darwin dreamed 

 of. " Even Darwin himself," Wallace says, " did not realise 

 how much and how universally wild species vary/' 



It is not to inherent variability, however, that the term 

 plasticity refers, but to modifiability, to the capacity of 

 being individually and directly changed in response to 



