38 ESSAYS OF A BIOLOGIST 



tually disadvantageous, 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 in- 

 efficient projectiles 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 in- 

 crease of all organisms leads to a "biological pres- 

 sure." 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 expand- 

 ing 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. Va- 

 riation towards such "low-pressure" regions may be 

 progressive, retrogressive, or neutral: but it is ob- 

 vious that at each stage of evolution there will al- 

 ways be a low-pressure fringe, representing a con- 

 siderable fraction of the "low-pressure" area within 

 the range of variability, the occupation of which 

 would be biologically progressive. 



Thus from the well-established biological premisses 

 of ( 1 ) the tendency to geometrical increase with con- 

 sequent struggle for existence, (2) some form of 

 inherited variability, we can deduce as necessary 

 consequence, not only the familiar but none the less 



