EVOLUTION 7^9 



that fundamentally classification remains a matter of associating 

 like things. 



NATURAL SELECTION 



Organic evolution could not become an important hypothesis 

 to account for the origin of species until some means was suggested 

 by which it could be reasonably supposed to have taken place. 

 Such a means was first convincingly argued by Darwin and 

 Wallace. Their theory of natural selection, or the survival of 

 the fittest, appeared to be logical, and indeed, on the facts as 

 they knew them, inevitable. It started from the observation, 

 first demonstrated for human populations by Malthus in 1802, 

 that man\^ more offspring are produced than can possibly survive. 

 This is most obvious for animals like fish or parasites which lay 

 millions of eggs in a year, but is also true of mammals and birds, 

 where each pair on an average produces more than two offspring, 

 yet the total numbers do not increase. To this observation was 

 added the phenomenon of variation — the fact that no two in- 

 dividuals are ever quite alike — and the deduction that, of a group, 

 those most fitted to live in a particular environment would 

 survive rather than those less fitted. ' Survival ' is here a relative 

 term ; what matters is the number of descendants which the 

 individual leaves, and the argument is that the better an animal 

 is suited to its environment, the longer it is likely to live and the 

 more descendants it is likely to leave. The theory was com- 

 pleted by adding a third observed generalisation — that like begets 

 like — and a usually concealed assumption that while the range 

 of variation would remain roughly the same, the mid-point 

 would gradually shift in the direction of the selection. If one 

 considers, for example, a carnivorous mammal feeding on mice, 

 it is obvious (that is, we assume) that the most agile individuals 

 will catch the most mice, and so be most successful, especially 

 in times of famine or when there are barely enough mice to go 

 round. They will therefore leave more descendants, and by 

 inheritance these will be more agile than the average, not only 

 in that none of them will be very sluggish, but also in that some 

 of them will be more agile than their parents. This will go on 

 generation after generation, and eventually the animals that we 

 call cats will be produced. At the same time selection will also 

 have operated on the mice, choosing those best able to escape 

 from the cats by speed, secretiveness or cunning. 



