6 Darwin, and after Darwin. 



hand, answers to discriminate isolation, or segregate 

 breeding : only individuals belonging to the same 

 variety or kind are allowed to propagate. Isolation, 

 then, is a genus, of which Apogamy and Homogamy 

 are species^. 



Now, in order to appreciate the unsurpassed im- 

 portance of isolation as one of the three basal 

 principles of organic evolution, let us begin by 

 considering the discriminate species of it, or Homo- 

 gamy. 



To state the case in the most general terms, we 

 may say that if the other two basal principles are 

 given in heredity and variability, the whole theory 

 of organic evolution becomes neither more nor less 

 than a theory of homogamy — that is, a theory of 

 the causes which lead to discriminate isolation, or 

 the breeding of like with like to the exclusion of 

 unlike. For the more we believe in heredity and 

 variability as basal principles of organic evolution, 

 the stronger must become our persuasion that dis- 

 criminate breeding leads to divergence of type, while 

 indiscriminate breeding leads to uniformity. This, 

 in fact, is securely based on what we know from the 

 experience supplied by artificial selection, which con- 



' I may here most conveniently define the senses in which all the 

 following terms will be used throughout the present discussion : — Species 

 of isolation are, as above stated, homogamy and, apogamy, or isolation 

 as discriminate and indiscriminate. Forms of isolation are modes of 

 isolation, such as the geographical, the sexual, the instinctive, or any 

 other of the numerous means whereby isolation of either species may be 

 secured. Cases of isolation are the instances in which any of the forms 

 of isolation may be at work : thus, if a group of n intergenerants be 

 segregated into five groups, a, b, c, d, e, then, before the segregation there 

 would have been one case of isolation, but after the segregation there 

 would be five such cases. 



