THBOTJGH CUMULATIVE SEGEEGATION. 



193 



organisms is subject follow the general law of " Frequency o£ 

 Deviation from an Average." As this is a law according to which 

 half of the members of the intergenerating group are above and 

 half below the average in relation to any character, there must 

 often occur simultaneous variation of several individuals in some 

 character which tends to produce Segregate Breeding. The 

 reality and importance of this law is not at all dependent on the 

 reality of any of the theories of heredity and variation that are 

 now being discussed. Whatever may be the causes that produce 

 variation, whether they depend entirely upon changes in external 

 conditions, or are chiefly due to changing activities in the 

 organism and the hereditary effects of acquired characters, or are, 

 as Weismann maintains, the direct result of sexual reproduction 

 which never transmits acquired characters, — in any and every 

 case this law of Deviation from an Average remains undisturbed, 

 and is recognized as an important factor in the present paper. 

 It therefore cannot be urged that the theory here advanced 

 assumes simultaneous variation without any ground for making 

 such an assumption ; nor can it be said that it rests on the in- 

 credible assumption that chance variation of very rare kinds will 

 be duplicated at one time and place, and will represent both 

 sexes. 



Moritz Wagner first discussed what he calls " The law of the 

 migration of organisms" in a paper read before the E/oyal 

 Academy of Sciences at Munich, in March 1868 ; but my attention 

 was not called to it till after the reading of my paper before the 

 British Association in August 1872. In a fuller paper entitled 

 "The Darwinian Theory and the Law of the Migration of 

 Organisms," an English translation of which was pubhshed by 

 Edward Stanford (London, 1873), the same author maintains that 



the constant tendency of individuals to wander from the 

 station of their species is absolutely necessary for the formation 

 of races and species " (p. 4). " The migration of organisms and 

 their colonization are, according to my conviction, a necessary 

 condition of natural selection " (p. 5). On pp. 66 and 67 he 

 expands the same statement, and objects to Darwin's view " that 

 on many large tracts all individuals of the same species have 

 become gradually changed." Again, he contends that " Trans- 

 formation is everywhere and always dependent on isolation in 

 order to have lasting efl'ect. Without separation from the home 

 of the species, this wonderful capacity would be completelj'' 



