12 ESSAYS OF A BIOLOGIST 



type has been sacrificed, and as end-product we are 

 presented with a hateful being, an almost shapeless 

 mass consisting of little else but over-developed re- 

 productive organs and mechanisms for sucking nutri- 

 ment from its unfortunate host. Such a result is 

 revealed to us in the Crustacean form Sacculina, 

 and is paralleled by countless other examples in almost 

 every class of animals. The degradation of parasites 

 and sedentary types is equally a product of the evolu- 

 tionary process with the genesis of the ant, the bird 

 or the human being ; how then can we call the 

 evolutionary process progressive ? 



These are important objections. Can they be 

 met ? In the broadest way they can and must be met 

 by the only possible method, the method of Science, 

 which consists in examining facts objectively, and by 

 drawing conclusions not a priori, but a posteriori. 

 A law of Nature is not (and I wonder how often this 

 fallacy has been exploded, only to reappear next day) 

 — 2i law of Nature is not something revealed, not 

 something absolute, not something imposed on pheno- 

 mena from without or from above ; it is no more 

 and no less than a summing-up, in generalized form, 

 of our own observations of phenomena ; it is an 

 epitome of fact, from which we can draw general 

 conclusions. By beginning in this way from the very 

 beginning, by examining the basis of our mode of think- 

 ing in natural science, only thus are we enabled to see 

 at one and the same moment how to investigate the 



