THE FACTOKS OF EVOLUTION 153 



remain alive more than two descendants of each pair of parents. 

 The oak or yew, the age of which is said to reach a thousand 

 years, need even produce only one germ in order to maintain 

 their species. As the rate of fertility of a species is generally 

 a fixed factor, it is easy to conclude from these two data how 

 many germs of life must be doomed to destruction if the normal 

 number is to be maintained. 



Of the six young of a pair of elephants it is therefore neces- 

 sary that four should die before they have reached maturity. 

 Considerably higher is the destruction in a pair of crows, for 

 supposing that crows hatch each year on an average twice, each 

 time five young, and continue to do so for twenty years, then 

 of these 200 descendants it is necessary that 198 die a premature 

 death. An even more terrible destruction we see in organisms 

 with high fertility. A carp, for instance, produces every year 

 about 250,000 eggs, and remains capable of reproduction for at 

 least fifty years. Of the 12,500,000 descendants, therefore, 

 12,499,998 perish as eggs, or embryos, or immature fish, for 

 only two must reach maturity to reproduce their species. If out 

 of the 100 million eggs produced by a sturgeon only one million 

 developed into mature females, and if this increase proceeded in 

 a similar manner through four generations, the fourth generation 

 would produce an amount of caviare greater than the entire 

 volume of the earth. But this wonderful event will, unfortu- 

 nately, never take place, for it is possible only for a very limited 

 number of individuals to exist ; all others are doomed to perish. 



Still more remarkable is the waste of life-germs in many para- 

 sitic worms, such as tape- worms, liver-flukes, &c., which produce 

 at one time up to 100 million eggs. In view of this fact it seems 

 almost inconceivable why parasites with such tremendous fertility 

 do not molest man and animals whom they infest to a far higher 

 degree, and why it is possible that such innumerable germs should 

 be destroyed almost as soon as they are born. But if we consider 

 the perilous life-cycle with its hundreds of dangers that beset 

 these parasites from the egg to the mature worm we shall under- 

 stand why among the many millions only a few favoured ones 

 are able to escape unscathed. The liver-fluke, Fasciola hepatica 

 (fig. 45), lives in the bile-ducts of sheep, cattle, pigs, &c., and 

 occurs sometimes in man. By stopping up the bile-ducts and 



