PROGRESS, BIOLOGICAL AND OTHER 31 



gressed; many have shown increase in one particular 

 but not in others. But the upper level of these prop- 

 erties 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 biolog- 

 ical 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 legiti- 

 mate way, as denoting a generalization based on ob- 

 served facts, and not as pre-supposing any vitalistic 

 principle of perfectibility, any necessary and mys- 

 terious tendency of organisms to advance independ- 

 ently of circumstances. 



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 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 



