98 HEREDITARY CHARACTERS 



laid to the time the fly produced from that egg is laying 

 eggs itself, is about three weeks. On these facts it has 

 been calculated, that if the progeny of one pair of flies 

 survived, producing offspring which in their turn survived, 

 and so on, a quarter of a million cubic feet of flies tightly 

 pressed together into a solid mass would be produced from 

 the single original pair in one summer. This is allowing 

 200,000 flies to the cubic foot, and assuming that an equal 

 number of males and females are produced. 1 But this is 

 not what happens. We find that the number of house-flies 

 does not vary to any great extent from year to year, so that 

 we must assume that all excepting one pair has been elimi- 

 nated. Every individual varies in some way from all the 

 other individuals ; it is extremely improbable that any two 

 are exactly alike. From the enormous masses of potential 

 individuals, natural selection has allowed only two survivals. 

 It has acted upon every stage from the egg to the adult 

 fly, and the survivors must have been among those most 

 accurately adapted to the condition under which they have 

 to live. The number from which this selection has been 

 made is so great, that except by some such illustration as 

 that given above, it is impossible to conceive it. There has 

 been then no lack of variations for selection to act upon. 



The female cod-fish lays from three to five million eggs 

 each year. She begins to lay eggs when about four years 

 old, and goes on laying for at least five or six years. What 

 would the numbers produced by one female cod and her 

 progeny be if any considerable proportion of them survived ? 

 We know that the number of cod is not increasing. The 

 numbers that natural selection has to act upon here are 

 just as inconceivable as in the case of the fly, and offer 

 ample material for the selection of minute favourable varia- 

 tions. The number of instances of this kind which might 

 be cited would fill many volumes. 



In the case of organisms that do not produce numerous 

 eggs or young, there is still amply sufficient material to 



1 " Biological Problems of To-day," Edinburgh Review, January 1909. 



