38 ESSAYS OF A BIOLOGIST 



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

 other, we have again a close parallel in the interrelated 

 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 actually dis- 

 advantageous, since it would have reduced its speed 

 relatively to other less heavily protected ships, without 

 conferring any corresponding benefit in the way of 

 defence against the comparatively inefficient pro- 

 jectiles of the day. Only when the range and piercing 

 power of the projectiles increased did increase of 

 armour become imperative. 



To resume our pressure analogy, the natural 

 increase of all organisms leads to a * biological pressure.' 

 So long as a species remains unchanged, so long must 

 it stay subjected to the full force of this pressure. 

 But if it changes in such a way that it can occupy a 

 new niche in environment, it is expanding into a 

 vacuum or a region of lower pressure. Natural 

 increase soon fills this up to the same level of pressure, 

 and conditions thus become favourable for expansion 

 into new low-pressure areas previously out of reach 

 of the normal range of variation. Variation towards 

 such ' low-pressure ' regions may be progressive, 

 retrogressive, or neutral : but it is obvious that at 

 each stage of evolution there will always be a low- 

 pressure fringe, representing a considerable fraction 

 of the ' low-pressure ' area within the range of vari- 



