32 ESSAYS OF A BIOLOGIST 



travelling fast may now be travelling slowly. None 

 the less, the average speed of all the particles is greater ; 

 and this and nothing else is what with perfect justifica- 

 tion we sum up as our law. 



In biological evolution, some organisms degenerate, 

 some remain stationary, but the average of certain 

 properties, and more especially their upper level, 

 increases ; and this tendency for certain properties 

 to become more marked, this and nothing else, is 

 what we sum up and generalize, again with perfect 

 justification, as the law of biological progress. 



The mechanism of biological progress demands a 

 word : for it is noticeable that a mere fact, how- 

 ever well attested, makes a very different kind of 

 impression from a fact explained and brought into 

 relation with the rest of our knowledge. The im- 

 pression is either less powerful ; or else, an explanation 

 being sought for, an erroneous one is found. It was 

 Darwin's great merit that, not content with the 

 piling up of evidence in favour of the reality of Evolu- 

 tion, he at the same time advanced a theory which 

 made it at least possible to understand how Evolution 

 could have come to pass as a natural process. The 

 effect was multiplicative on men's minds, not merely 

 additive, for facts are too bulky to be lugged about 

 conveniently except on wheels of theory. 



The fact of biological progress has struck many 

 observers. Some have been content to believe that 

 the single magic formula of * Natural Selection ' 



