y]^ Evolution by ]umps 



how a new heritable trait can suddenly appear in a healthy 

 stream of protoplasm. And yet something in the way of 

 discontinuity is implied by the general theory of evolution, 

 and by the facts. The paradox is similar to that propounded 

 by the Greek skeptic Zeno: Motion is impossible because a 

 thing cannot move where it is, for so long as it is there it is 

 at rest; and it cannot move where it isn't, for it isn't there 

 to do anything. And yet we know that things do move. 



It is commonly assumed by many that the idea of evolu- 

 tion corresponds rather to a continuity of events and proc- 

 esses. The suggestion of discontinuity to explain evolution 

 therefore appears self-contradictory. There is, however, no 

 fundamental inconsistency. The continuous pressure of 

 water in a pipe can furnish a continuous flow at the open end, 

 under ordinary conditions. If the opening is closed down 

 sufficiently, however, but not too much, we can get from the 

 same continuous pressure a series of interrupted discharges 



— as in the case of a dripping faucet. This is only to say 

 that a continuous force can be converted into a periodic or 

 interrupted phenomenon. There is no intention of suggest- 

 ing how discontinuity arises in protoplasmic heredity. 



Islatura Non Facit Saltuvt 



Long before Weismann's time it was recognized among 

 biologists that the facts which point to an evolution of plant 

 and animal forms point also to some break in the continuity 

 of heredity and generation. Huxley himself was a stout 

 defender of Darwinism and a tireless promoter of the doc- 

 trine of evolution, yet in his review of The Origin of Species, 

 shortly after the book came out, he noted that Darwin might 

 have made out a stronger case for evolution if he had not 

 committed himself to the maxim, Natura non facit saltum 



— Nature makes no jumps. Francis Galton, in his work on 

 Natural Inheritance, saw no need for insisting upon the 

 steps in evolution being too small. He saw the difficulty that 

 if each step were very small, a new character would be of no 



