PROBLEMS OF BIOLOGY 323 



And, as I have said, I believe that the forces of surface- 

 tension, elasticity, and pressure are adequate to account for 

 a great multitude of the simpler phenomena, and the per- 

 mutations and combinations thereof, that are illustrated in 

 organic form. 



I might well have devoted this essay to these questions, 

 and to these alone. But I was loath to do so, lest I should 

 seem to overrate their importance, and to appear to you as 

 an advocate of a purely mechanical biology. I believe all 

 these phenomena to have been unduly neglected, and to call 

 for more attention than they have received; but I know 

 well that though we push such explanations to the uttermost, 

 and learn much in the so doing, they will not touch the heart 

 of the great problems that lie deeper than the physical 

 plane. Over the ultimate problems and causes of vitality 

 we shall be left wondering still. 



To a man of letters and the world like Addison, it came 

 as a sort of revelation that light and colour were not objec- 

 tive things but subjective, and that back of them lay only 

 motion or vibration, some simple activity. And when he 

 wrote his essay on these startling discoveries, he found for 

 it, from Ovid, a motto well worth bearing in mind causa 

 latet, vis est notissima. We may with advantage recollect 

 it, when we seek and find the Force that produces a direct 

 Effect, but stand in utter perplexity before the manifold and 

 transcendent meanings of that great word Cause. 



The similarity between organic forms and those that 

 physical agencies are competent to produce, still leads some 

 men, such as Stephane Leduc, to doubt or to deny that there 

 is any gulf between, and to hold that spontaneous genera- 

 tion or the artificial creation of the living is but a footstep 

 away. Others, like Delage and many more, see in the 

 contents of the cell only a complicated chemistry, and in 

 variation only a change in the nature and arrangement 

 of the chemical constituents ; they either cling to a belief in 



