22 CONDITIONS AFFECTING THE 



L,i<w 



as direct helpers to the tape-worm, but the fact is so : 

 we can all see that if there were no men there would 

 be no tape-worms. 



It is extremely difficult to estimate, in a proper way, 

 the importance and the working of the Conditions of 

 Existence. I do not think there were any of us who 

 had the remotest notion of properly estimating them 

 until the publication of Mr. Darwin's work, which 

 has placed them before us with remarkable clearness; 

 and I must endeavour, as far as I can in mv own 

 fashion, to give you some notion of how they work. 

 AVe shall find it easiest to take a simple case, and one 

 as free as possible from every kind of complication. 



I will suppose, therefore, that all the habitable part 

 of this globe — the dry land, amounting to about 

 51,000,000 square miles, — I will suppose that the whole 

 of that dry land has the same climate, and that it is 

 composed of the same kind of rock or soil, so that 

 there will be the same station everywhere; we thus 

 get rid of the peculiar influence of different climates 

 and stations. I will then imagine that there shall be 

 but one organic being in the world, and that shall be a 

 plant. In this we start fair. Its food is to be carbonic 

 acid, water and ammonia, and the saline matters 

 in the soil, which are, by the supposition, everywhere 

 alike. We take one single plant, with no oppo- 

 nents, no helpers, and no rivals ; it is to be a " fair 

 field, and no favour." Now, I will ask you to imagine 

 further that it shall be a plant which shall produce 

 every year fifty seeds, which is a very moderate num- 

 ber for a plant to produce; and that, by the action 

 of the winds and currents, these seeds shall be equally 



