xxxii. 16 INFLUENCES IN EVOLUTION 783 



variables. J. B. S. Haldane, R. A. Fisher, Sewall Wright, and others 

 have made some progress towards analysing this situation into its 

 elements by mathematical reasoning, but we have at present estimates 

 for only few of the variables involved and no approach to an exact 

 solution is possible. 



16. Conservative and radical influences in evolution 



In such a system we have factors that tend to keep the population 

 the same and others that tend to make it vary; according to the pre- 

 ponderance of one or the other the characters of the populations will 

 either stay steady about a mean or tend continually to change, that is 

 to say, to evolve. We cannot make any very precise list of the factors in 

 the two classes. Those tending to reduce evolutionary change presum- 

 ably include (1) The copying or reproductive tendency that makes like 

 produce like. Here we may include the restraints imposed by the fact 

 that development is a highly integrated process, so that any wide 

 departures from normality are liable to interfere with it. (2) Anything 

 that reduces the productivity in terms of number of offspring. 



(3) Prevalence of predators, if these act so as to prohibit minor devia- 

 tions from the previous mode of life. (4) Absence of geographical 

 barriers. (5) Existence of stable external climatic and other physical 

 factors. (6) Characteristics within the population tending to keep the 

 animals in their existing conditions, rather than to seek new ones; 

 'adventurousness' is essential if the individuals are to enter new con- 

 ditions; it corresponds to the adaptations for dispersal that are found 

 among plants (Salisbury, 1942). 



The contrary circumstances, those that encourage evolutionary de- 

 velopment, are presumably (1) High rate of mutation, that is to say, 

 failure of the tendency to copy. Spurway has pointed out that the 

 extent to which change in the hereditary and developmental pattern 

 can be varied probably differs in different populations. In some almost 

 any change is lethal and the population appears immutable, whereas 

 other organizations can stand considerable variations, allowing the 

 possibility of rapid evolution. (2) High productivity, if it leads to high 

 intra-specific competition and hence pressure to find new conditions 

 for life. (3) Absence of predators, if this makes new adventures possible. 



(4) Presence of geographical barriers, which allow separation into 

 various races that do not interbreed (or only seldom do so) and hence 

 become more and more distinct. (5) Change in external conditions. 

 (6) Presence of a high power of adaptability and 'adventurousness' in 

 the various action systems of the animals. (7) Haldane has suggested 



