ECOLOGY AND EVOLUTION i8i 



Looked at from another angle, the process of formation of 

 a new species can be divided up into three phases : 

 (i) Occurrence of a genotypic (heritable) variation. 



(2) Spread of this variation in the population. 



(3) Isolation of this new stock so as to form, ultimately, a 



new species. 

 The process of isolation does not necessarily come in in 

 all cases, however, since a new variety might simply spread 

 through the whole population of that species, and automatically 

 change the w^hole stock. 



3. With the origin of genotypic variations we are not here 

 concerned. Ecology has, however, a definite contribution to 

 make towards the study of the second and third phases. The 

 usual Darwinian assumes that a variation which crops up 

 singly, or at any rate rarely, has absolutely no chance of spread- 

 ing in the population unless it is favoured by possessing some 

 advantage over its fellows. This argument appears at first 

 sight irrefutable. If a cod has a million eggs, and one of these 

 eggs contains a new hereditary factor, what chance has this 

 particular egg of growing up to the one of the two successful 

 cods out of that million ? And if it did reach the breeding 

 stage, and had a million young itself, only one of these, if any, 

 would survive in the next generation. The Darwinian 

 assumes that the deadly chances against any new variation 

 spreading to any extent in the population can only be wiped 

 out by the favourable influence of natural selection. If this 

 is so, then all the characters possessed by animals — at any rate 

 those which separate closely allied species — must either be of 

 some direct use to the species (or to one sex in the species), or 

 else they must owe their existence to the fact that they are 

 intimately bound up in development with some other character 

 which is useful, e.g. both might be products of the same 

 hereditary factor in the egg. We see, then, that one of the 

 great arguments in favour of the natural selection theory is the 

 difficulty of any other hypothesis about the spread of variations, 

 once they have arisen ; while another argument is that all 

 animals are simply masses of adaptations. 



4. So far, we have been arguing from one step to the next, 



