How Darwinism Stands To-Day 371 



but the great majority of the deviations will be not far from the 

 mean. 



Definiteness in Variation 



Since Darwin's time evidence has accumulated which shows 

 that variations are more definite than used to be supposed. The 

 palaeontologists, who work out long series of fossils, bring for- 

 ward cases of what looks like steady progress in a definite direc- 

 tion. There is a striking absence of what one might call arrows 

 shot at a venture. It looks as if the occurrence of the new were 

 limited by what has gone before, just as the architecture of a 

 building that has been erected determines in some measure the 

 style of any addition. An organic new departure will tend to 

 be more or less congruent with what has been previously estab- 

 lished. In post-Darwinian days the element of the fortuitous 

 has shrunk. 



Discontinuous Variations 



Darwin was much interested in sports or freaks, such as the 

 sudden appearance of a dwarf or a giant, a hornless calf or a 

 tailless kitten, a v/hite blackbird or a weeping ash, a thornless 

 rose or a stoneless plum, a "wonder-horse" with its mane reach- 

 ing the ground, or a Japanese cock with a tail six feet long. 

 But Darwin did not venture to attach great importance to these 

 brusque novelties, or discontinuous variations, first because he 

 thought they were of rare occurrence, and second because he 

 thought they would be speedily averaged off in the offspring of 

 a sport which had paired with an ordinary individual. He did 

 not know what his contemporary Mendel proved, that when a 

 pure-bred tall pea and a pure-bred dwarf pea are crossed the 

 offspring are all tall. 



Now one of the great changes that has come about since 

 Darwin's day is a recognition of the frequency of discontinuous 

 variations, by which we mean sudden novelties which are not 



