REASON OP ELEVATED FECUNDITY-RATE 7 



caprice of Nature. Take the case of the tape-worm, Tcenia 

 solium, which produces 100,000,000 eggs. In order that one of 

 these eggs may be developed, it has to survive an astonishing 

 number of risks. The joints of the tape-worm, laden with 

 developing eggs, have to pass out of the human food-canal in 

 which the adult tape-worm Uves. The joints have to liberate 

 the embryos by bursting, and the embryos have to be swallowed 

 by a pig before they can develop further. The embryos liberated 

 in the pig's stomach penetrate the wall of the food-canal, and 

 bore their way into the muscles — ^it may be to some remote part 

 of the pig's body. There the embryo goes through a further 

 period of development, becoming a bladder-worm ; but, in order 

 that the development may be completed, it is necessary that 

 the pig should first die, and, finally, chance must so ordain 

 matters that a piece of its flesh containing the bladder-worm is 

 eaten by some mammal or by man. Only in the latter case, 

 when the bladder- worm reaches the human intestine, will it be 

 able to develop into a full-grown tape-worm. One can under- 

 stand how extremely rarely all these vicissitudes are survived, 

 and how necessary it is for the stability of the Tcenia species 

 that its fecundity-rate should be so enormously high. 



A consideration of the excessive fecundity of all life brings 

 us to the fourth factor in the Darwinian doctrine of descent — 

 that of selection. When we consider the enormous death-rate 

 throughout the whole range of species ; when we reflect that, out 

 of the hundreds or thousands of progeny which may compose a 

 family, on an average only two survive and reproduce ; the ques- 

 tion must suggest itself, Does the survival of the two in question 

 depend upon chance ? What mysterious mechanism is it which 

 ordains that, on an average, only two of aU these ofispring sur- 

 vive ? And, to go further, why do the two which survive, sur- 

 vive ? Why not other two ? 



Darwin has given the answer to these questions in terms of 

 natural selection and its corollary, the survival of the fittest. 



