THE DOGMA OF EVOLUTION 



than a year after his return : "I opened my first note- 

 book for facts in relation to the Origin of Species, 

 about which I had long reflected, and never ceased 

 working for the next twenty years." He married in 

 1842 and, after living in London for nearly four 

 more years, he moved to Down on account of ill health 

 and lived there the rest of his life, a confirmed invalid. 

 Although his belief in evolution was fixed, and he 

 had determined to make its verification his life-work, 

 he remained for a year unable to find a cause for a 

 variation which would progressively change a species 

 into a new one. The clue came to him in 1838, while 

 reading Mai thus, whose theory of population was 

 then having its greatest vogue. He immediately 

 adopted the idea of this writer that population tends 

 to increase geometrically while the food supply can 

 increase only arithmetically.^ Thus, Malthus reasons 



1 The difference between a geometrical and an arithmetical progres- 

 sion can be best explained by the statement that a geometrical series 

 of numbers is one in which each succeeding number changes by a 

 constant multiplier and that an arithmetical series changes by a 

 constant factor of addition or subtraction. For example, let 2 be 

 the constant of such series. Then we have for a geometrical series 

 the numbers 2, 4, 8, 16, 32, 64, etc., and for an arithmetical series 

 2, 4, 6, 8, 10, 12, etc. It is thus evident that a geometrical series in- 

 creases much faster than an arithmetical series and no matter how 

 small the multiplying factor may be and how large the factor of 

 addition may be taken, if sufficient terms are taken, the end term 

 of a geometrical series is necessarily greater than that of an arith- 

 metical series. Malthus's idea was that eadh two parents will have, 

 on an average, more than two children who reach maturity and leave 

 offspring; for example, let us say four. Population will then double 

 in each generation. On the other hand, by increase of land under 

 cultivation and by other means, the food supply in any country 

 can increase only arithmetically. Thus, under any constants of the 

 two series, population is bound in time to pass the available food 

 supply. , 



n 186 2 



