30 HEREDITY 



that actual changes in the nature of the germ plasm are 

 comparatively rare. They are the exception and not 

 the rule in heredity. 



It is impossible in our small space to discuss aU the 

 reasons on wliich this idea is based, but some of them 

 wiU emerge in the sequel. 



This conception of variation is very different from 

 that formed by Darwin, who studied the subject very 

 thorouglily. Dar\\'in beheved that inborn variation was 

 the rule ; that the characters of a species were in a 

 state of flux, so that new variations in one direction or 

 the other could be produced merely by selecting in those 

 directions. One sj)ecies was formed from another by 

 the accumulation of minute indefinite tendencies in one 

 direction. Dar^^dn emphasised the slowness of the pro- 

 cess of change in hving tilings ; Natura non fecit salium 

 (" Nature does not leap ") was a favourite aphorism of 

 his. The process of change is no doubt slow in the mass ; 

 but we have lately come to the opinion that nature 

 does leap — in fact, that it is only by leaps that change 

 can occur. The slo\vness of evolution, then, is due to 

 the fact that the leaps are seldom and smaU. 



It is perhaps necessary to say that mutations may 

 be either progressive or retrogressive. The change may 

 be a gain of a new character, but is perhaps oftener a 

 loss of a character wliich had been gained prev^iously 

 in the history of the race. 



Any variation in which a characteristic of a remote 

 ancestor recurs is termed a reversion. Certain rever- 

 sions are due to special cases of recombination, which 

 will be discussed later. But some reversions at least 

 are retrogressive mutations. 



As regards the causes of mutations it must be ad- 

 mitted that we are still in almost complete ignorance. 

 We have had enough of speculation on the matter, but 

 the cases in which we can point to any definite cause 

 are extremely few. Even when a connection has been 

 observed between a certain condition and the occurrence 

 of a mutation, it has been impossible to trace the chain 

 of causes connecting the two. This is probably due to 



