PROGRESS, BIOLOGICAL AND OTHER 37 



case both of man-made machine and evolving verte- 

 brate group, there is first a sudden increase in num- 

 bers 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 possessors to survive : it is not the best pos- 

 sible 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 ex- 

 istence as dominant types. 



To the interrelated evolution of carnivore and 

 herbivore, again, leading to increase of size and speed 

 in both, of wariness in one, of tooth and claw in the 

 other, we have again a close parallel in the interre- 

 lated evolution of armour-plating and of projectiles. 

 Here again the process is gradual. We can further 

 see that the sudden ''development" of full modern 

 armour on the first iron-clad would have been ac- 



