2 INHERITANCE OF COAT-PIGMENTS AND COAT-PATTERNS 



both sorts in approximately the same proportions in which they occur in 

 the jar. The result is a "chance" one, but controlled by a perfectly definite 

 mathematical law. 



A "chance result" has been aptly defined as the result of a number of 

 causes acting independently of each other. If this is a valid definition, 

 then a continuous series of variations is due to no single cause but to several 

 mutually independent ones. Some of the causes may be external in origin, 

 others internal; some temporary in their action, others permanent. It 

 should not surprise us, therefore, to find that continuous variations differ 

 greatly in the degree of their inheritance. De Vries, indeed, has maintained 

 that they are not inherited at all, except temporarily; that selection of 

 abmodal variations from a continuous series is unable permanently to modify 

 a race ; that the modifications will persist only so long as selection continues, 

 but will speedily disappear when selection is arrested. This conclusion, 

 however, seems to us altogether too sweeping. A priori there is no reason 

 to suppose that all the causes operative to produce continuous variation 

 are external in origin and temporary in action, as De Vries's conclusion 

 would seem to imply. If there are in operation, in the production of a 

 continuous series of variations, causes internal in origin, resident in the 

 constitution of the germinal substance, so much of the result as is due to 

 those causes should be inherited and so should be permanent. De Vries, we 

 believe, has overlooked this factor entering into the problem. He has 

 assumed that all the causes of continuous variation ("fluctuations") are 

 either external in origin or due to conditions of the germinal substance 

 purely temporary. He holds, we believe rightly, that all inheritance is due 

 to germinal modification ; but assumes, we believe without sufficient warrant, 

 that permanent germinal modification is not a factor in the production of 

 fluctuations. 



Another category of variations, discontinuous variations (which include 

 the mutations of De Vries), is considered by Bateson and De Vries as the 

 true and only expression of permanent germinal modification. But, grant- 

 ing the truly germinal origin of mutations, it does not follow that they are 

 the only product of germinal modification. 



A discontinuous variation, as the name suggests, is unconnected by inter- 

 mediate conditions with the usual (modal) condition of the species. It 

 represents a change, more or less abrupt, from the modal condition of the 

 species, and is strongly inherited, a fact which indicates clearly its exclu- 

 sively germinal origin. 



In the category of discontinuous variations belong abrupt changes in 

 pigmentation and hairiness among both animals and plants, changes in the 

 number of digits or of the number of phalanges in a digit among vertebrates, 

 in the presence or absence of horns among animals and spines among plants, 

 and other similar conditions. 



