32 ESSAYS OF A BIOLOGIST 



with perfect justification we sum up as our law. 



In biological evolution, some organisms degener- 

 ate, some remain stationary, but the average of cer- 

 tain 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 justifi- 

 cation, as the law of biological progress. 



The mechanism of biological progress demands a 

 word: for it is noticeable that a mere fact, however 

 well attested, makes a very different kind of impres- 

 sion from a fact explained and brought into relation 

 with the rest of our knowledge. The impression is 

 either less powerful; or else, an explanation being 

 sought for, an erroneous one is found. It was Dar- 

 win's great merit that, not content with the piling up 

 of evidence in favour of the reality of Evolution, 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 ad- 

 ditive, for facts are too bulky to be lugged about 

 conveniently except on wheels of theory. 



The fact of biological progress has struck many ob- 

 servers. Some have been content to believe that the 

 single magic formula of "Natural Selection" would 

 explain it adequately and without further trouble, for- 

 getting that there must be at least some points of 

 difference between a natural selection producing a 

 degenerate type and natural selection leading to prog- 



