12 ESSAYS OF A BIOLOGIST 



which the activity and complex organization of 

 the ancestral type has been sacrificed, and as end- 

 product we are presented with a hateful being, an al- 

 most shapeless mass consisting of little else but over- 

 developed reproductive organs and mechanisms for 

 sucking nutriment from its unfortunate host. Such 

 a result is revealed to us in the Crustacean form Sac- 

 culina, 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 evolutionary 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 objec- 

 tively, 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 re- 

 appear next day) — a law of Nature is not something 

 revealed, not something absolute, not something im- 

 posed on phenomena from without or from above; it 

 is no more and no less than a sumrning-up, in gener- 

 alized form, of our own observations of phenomena; 

 it is an epitome of fact from which we can draw sev- 

 eral conclusions. By beginning in this way from 

 the very beginning, by examining the basis of our 

 mode of thinking in natural science, only thus are 

 we enabled to see at one and the same moment how 



