THROUGH CUMULATIVE SEGREGATION. 
193 
organisms is subject follow the general law of “ Frequency of 
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, aud will represent both 
sexes. 
Moritz Wagner first discussed what he calls “ The law of the 
migration of organisms ” in a paper read before the Royal 
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 published 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 87 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 effect. Without separation from the home 
of the species, this wonderful capacity would be completely 
