PROGRESS 31 



This increase has not been universal ; many organ- 

 isms have remained stationary or have even regressed ; 

 many have shown increase in one particular but not 

 in others. But the upper level of these properties 

 of living matter has been continually raised, their 

 average has continually increased. It is to this 

 increase, continuous during evolutionary time, in the 

 average and especially in the upper level of these 

 properties that, I venture to think, the term biological 

 progress can be properly applied. 



Used thus it is no more an a priori or an undefined 

 concept. It is a name for a complicated set of actual 

 phenomena, and if, with progress thus defined, we 

 were to speak of a law of progress in evolution, we 

 should be using the term law in a perfectly legitimate 

 way, as denoting a generalization based on observed 

 facts, and not as pre-supposing any vitalistic principle 

 of perfectibility, any necessary and mysterious tendency 

 of organisms to advance independently of circum- 

 stances. 



The gas laws state that the pressure of a gas kept 

 at constant volume increases in a particular way with 

 increase of temperature. Now the pressure of a con- 

 fined gas depends on the rate at which its particles 

 bombard the walls in which they are contained, and 

 the speed at which they are travelling. In a gas whose 

 temperature is raised, many particles will, at any given 

 moment, be travelling more slowly than the average 

 rate when it was cooler, many even which had been 



