Variation and Natural Selection. 77 



and the many marine forms which produce a great number 

 of fertilized eggs giving rise to embryos that are from an 

 early period free-swimming and self-supporting. Such 

 embryos are decimated by a destruction which is quite 

 indiscriminate. And again, to take but one more example, 

 the liver-fluke, whose life-history was sketched in the last 

 chapter, produces its tens or hundreds of thousands of 

 ova. But the chances are enormously against their com- 

 pleting their life-cycle. If the conditions of temperature 

 and moisture are not favourable, the embryo is not hatched 

 or soon dies ; even if it emerges, no further development 

 takes place unless it chances to come in contact with a 

 particular and not very common kind of water-snail. When 

 it emerges from the intermediate host and settles on a 

 blade of grass, it must still await the chance of that blade 

 being eaten by a sheep or goat. It is said that the chances 

 are eight millions to one against it, and for the most part 

 its preservation is due to no special excellence of its own. 

 The destruction is to a large extent, though not entirely, 

 indiscriminate. 



Even making all due allowance, however, for this indis- 

 criminate destruction which is to a large extent avoided 

 by those higher creatures which foster their young there 

 remain more individuals than suffice to keep up the normal 

 numbers of the species. Among these there arises a 

 struggle for existence, and hence what Darwin named 

 natural selection. 



" How will the struggle for existence "I quote, with 

 some omissions, the words of Darwin " act in regard to 

 variation? Can the principle of selection, which is so 

 potent in the hands of man, apply under nature ? I think 

 that we shall see that it can act most efficiently. Let the 

 endless number of slight variations and individual differ- 

 ences be borne in mind ; as well as the strength of the 

 hereditary tendency. Let it also be borne in mind how 

 infinitely complex and close-fitting are the mutual relations 

 of all organic beings to each other and to their physical 

 conditions of life ; and consequently what infinitely varied 



