PERPETUATION OF LIVING BEINGS. 117 



same kind of station, or food, or climate ; those are the 

 indirect opponents ; the direct opponents are, of course, 

 those which prey upon an animal or vegetable. The 

 helpers may also be regarded as direct and indirect : in 

 the case of a carnivorous animal, for example, a particu- 

 lar herbaceous plant may in multiplying be an indirect 

 helper, by enabling the herbivora on which the carni- 

 vore preys to get more food, and thus to nourish the 

 carnivore more abundantly ; the direct helper may be 

 best illustrated by reference to some parasitic creature, 

 such as the tape- worm. The tape-worm exists in the 

 human intestines, so that the fewer there are of men 

 the fewer there will be of tape-worms, other things 

 being alike. It is a humiliating reflection, perhaps, 

 that we may be classed 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 my own fashion, to 

 give you some notion of how they work. We shall find 

 it easiest to take a simple case, and one as free as possi- 

 ble from every kind of complication. 



I will suppose, therefore, that all the habitable part of 

 this globe — the dryland, 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 



