THE DARWINIAN THEORY 47 



Leuwenhoek calculated the fertility of a thread-worm at sixty 

 million eggs, and a tape-worm produces hardly less than loo millions. 



There exists, therefore, a constant relation between fertility and 

 the ratio of elimination ; the higher the latter is, the greater must the 

 former be, if the species is to survive at all. The example of the 

 tape-worm makes this very obvious, for here we can readily under- 

 stand why the fertility must be so enormous, as we are aware of the 

 long chain of chances on which the successful development of this 

 animal depends. The common tape-worm of Man, Taenia solium, 

 does not lay its eggs, they remain enclosed within one of the 

 liberated joints or ' proglottides.' Only if this liberated joint or one 

 of the embryos within it happens to be fortuitously eaten by a pig or 

 other mammal can there be successful development, and even then 

 under difficulties and possible failures, and not right away into adult 

 animals, but first into microscopically minute larvte which may bore 

 their way into the walls of the intestine, or, if they are fortunate 

 enough, may get into the blood-stream and be carried by it to 

 a remote part of the body. There they develop into 'measles,' the 

 so-called bladder-worms, within which the head of the tape-worm 

 arises. But in order that this may become a complete and reproduc- 

 tive adult worm the pig must die, and the next step necessary is that 

 a piece of the flesh of the infected first host must happen to be 

 swallowed raw by a man or other mammal ! Only then does the 

 fortunate bladder- worm — swallowed with the fiesh — attain the goal 

 of its life, that is, a suitable place to mature in, the food-canal of 

 a human being. It is obvious that countless eggs must be lost for 

 one that succeeds in getting through the whole course of a develop- 

 ment depending so greatly on chance. Hence the necessity for such 

 enormous productivity of eggs. 



In many cases the causes of elimination, which keep a species 

 within due bounds, are very difficult to determine. Enemies, that is 

 to say, other species which use the species in question as food, play an 

 important r61e ; often, however, the cause lies in the unf avourableness 

 of external conditions, in chance, which is favourable only to one of 

 a thousand. The oak would only require to produce one seed in the 

 500 years of its life, if it were certain that that one would grow into 

 an oak-tree ; but most of the little acorns are eaten up by pigs, squirrels, 

 insects, &c., before they have had time to sprout, thousands fall on 

 ground already thickly covered with growth where they cannot take 

 root, and even if they do succeed in finding an unoccupied space in 

 which to germinate, the young plants are still surrounded by a thousand 

 dangers— the possibility of being devoured by many animals large and 



