The Source of Variability 61 



and then became extinct; that other hnes seemingly arose and sup- 

 planted older groups, as the birds and mammals seem to have sup- 

 planted the dinosaurs; but that, by way of contrast, other groups 

 such as cockroaches and molluscs, already old when the dinosaurs 

 were young, are even now successfully continuing several hundred 

 million years of existence. Many of the marine Mollusca and uni- 

 cellular organisms afford a sui-prising number of examples of these 

 long-lived types. Examining the composition of living categories, 

 we note that some genera contain only one or two species, whereas 

 other genera of comparable age may have a thousand species. 

 These conditions of unequal evolutionary development in different 

 phylogenetic lines do not fit a picture of the evolution of life from 

 the accumulation of equal radial paths of variation. Rather, the 

 situation indicates some influence or combination of influences that 

 results in a drastic deviation from either the production or the 

 survival of an orderly, automatic radiation of life. 



All the known evidence indicates that these influences comprise 

 two great sets of factors. One set constitutes the origin of new 

 variants. Its causes arise within the hereditary mechanism of the 

 organism, including both changes in the genetic material itself and 

 recombinations of these changes. The other set of factors involves 

 the fate of the variants. It includes effects of both the internal and 

 external environments which in one way or another determine 

 which genetic types survive. 



In the past several relatively simple explanations have been given 

 for both sets of factors. The more these phenomena are studied, 

 however, the more difficult it becomes to find a satisfactory ex- 

 planation for how the hereditary mechanism works, or how it 

 changes, or how the more selective factors are identified in the 

 environment. One fact, however, has been confirmed by almost all 

 of the experiments performed. That part of the external environ- 

 ment (such as weather, predation, and food) which exerts a selec- 

 tive action on genetic types, has at most a very limited effect in 

 causing genetic mutation. It seems that in almost all instances 

 genetic variants arise as some function of the individual independ- 

 ent of its environment. Whatever their origin, however, only after 

 spontaneous changes have occurred in the individual can the en- 

 vironment influence their survival. 



It is therefore pertinent to consider first the spontaneous phe- 

 nomena associated with the production of variation, in other words, 

 the material basis of evolution. Primarily this involves the identifica- 



