82 HEREDITY AND VARIATION 



We know also that many domestic races have, as a matter of 

 fact, arisen by sudden mutation. 



As to the series of related species which may be often arranged 

 as if on an inclined plane, two points should be noted : 

 (i) that it is likely enough that some kinds of species, e.g. vege- 

 tative forms like Alcyonarians and Corals, may have evolved by 

 minute steps, and (2) that although species are often connected 

 by intermediate links it does not follow that these links are 

 stages in the evolution. They may have been formed after the 

 species to which they are theoretically supposed to give rise. 

 We should remember Galton's warning, " If all the variations of 

 any machine that had ever been invented were selected and 

 arranged in a museum, each would differ so little from its neigh- 

 bours as to suggest the fallacious inference that the successive 

 inventions of that machine had progressed by means of a very 

 large number of hardly discernible steps." Many facts now 

 lead us to conclude that the Proteus leaps as well as creeps. 



§ 6. Discontinuous Variations 



One of the steps of progress in Evolution- lore since Darwin's 

 day is the recognition of the frequency and importance of dis- 

 continuous variations — i.e. of organic changes which arise 

 abruptly and not by a gradual series of steps. If dwarfs arise 

 suddenly in a tall race, and are not mere modifications, they 

 illustrate discontinuous variation of a quantitative sort. A 

 hornless calf, a tail-less kitten, a short-legged lamb, a thornless 

 rose, illustrate discontinuous quantitative variations of a negative 

 kind. Giants, " wonder-horses," long-tailed Japanese cocks, 

 merino-fleeced sheep, spine-covered holly leaves illustrate dis- 

 continuous quantitative variations of a positive kind. Sometimes 

 the novelty cannot be readily expressed in quantitative terms — 

 an entirely new colour turns up, the variant is immune to certain 

 diseases to which the stock is susceptible, leaves become fasciated, 



