PROGRESS 37 



the new type is in a precarious and experimental stage, 

 only just managing to exist, of small size and rare 

 occurrence, and in no real sense a serious rival to 

 its old-established competitors. Then, suddenly, a 

 change. It reaches a level at v^^hich it can effectively 

 compete with them. What happens ? In the case 

 both of man-made machine and evolving vertebrate 

 group, there is first a sudden increase in numbers of 

 the new, a corresponding decrease in numbers of the 

 old type. The upper level of size of the new type 

 also begins to increase, and it begins to split up into 

 a great number of differentiated sub-types. Some of 

 these sub-types become extinct, others, on the other 

 hand, are gradually improved, while still others undergo 

 such rapid change as to merit the style of new sub- 

 types. The upper level of size, complexity, and 

 efficiency increase, both in animal and machine. 



It is as well to remember that survival-value means 

 only what it says. A variation with survival-value 

 helps its possessor to survive : it is not the best possible 

 variation of the kind. In the developing motor-car, 

 the substitution of four for one or two cylinders was a 

 great improvement. It had 'survival value' ; and not 

 until the majority of cars came to be four-cylindered 

 was the additional advantage of six or eight cylinders 

 large enough to bring them into existence as dominant 

 types. 



To the interrelated evolution of carnivore and 

 herbivore, again, leading to increase of size and speed 



