SELECTION AND CROSS-BREEDING IN RELATION TO 

 THE INHERITANCE OF COAT-PIGMENTS AND COAT- 

 PATTERNS IN RATS AND GUINEA-PIGS. 



By Hansford MacCurdy and W. E. Casti,b. 



CONTINUOUS VERSUS DISCONTINUOUS VARIATIONS AS 

 FACTORS IN EVOLUTION. 



It is generally agreed that the course of evolution is largely influenced 

 by two factors, variation and heredity; but opinions differ as to what sorts 

 of variation have evolutionary significance and as to the manner of their 

 inheritance. 



It has been recognized by several investigators that variations are of 

 two distinct sorts. Bateson has called these two sorts of variation contin- 

 uous and discontinuous ; more recently De Vries has called them fluctuations 

 and mutations, respectively. 



By continuous variation (or fluctuation) we understand ordinary indi- 

 vidual variation within a species. The individuals differ among themselves 

 in size, color, and other structural features. By examining a considerable 

 number of them we can form an idea of what is the commonest (or modal) 

 condition as regards each structural feature ; and likewise what is the average 

 (or mean) condition. 



Usually, but not always, the modal and mean conditions are approximately 

 the same, and any other condition is the less frequent in occurrence, the 

 greater its deviation from them. It follows that the most extreme condi- 

 tion observed is connected with the most usual (or modal) condition by 

 an unbroken series of intermediate conditions, and we may call the series 

 as a whole "continuous." The distribution of the individuals in such a 

 series is governed by the laws of "chance," and may be successfully 

 analyzed by statistical methods. 



We commonly think of a "chance" result as something entirely beyond 

 the control of law, but in reality such is not the case. Nothing is beyond 

 the control of law. If a blindfolded person puts his hand into an urn con- 

 taining a mixture of black and of white balls, it is a matter of chance whether 

 he grasps a black or a white ball; but if he repeats the operation a con- 

 siderable number of times, it is perfectly certain that he will draw balls of 



