250 



PEESIDENTIAL ADDEESS. 



the individuals born : a, a^, etc., represent the individuals 

 above the average : b, b^, etc., represent the individuals below 

 the average. The individuals represented by 6, &\ etc., are 

 eliminated through the struggle for existence in each 

 generation. There survive in each generation a, a}, etc., 

 and m, vi^, etc. ; and these interbreed. Take, for example, 

 the first group a, m, b: b are eliminated : a and 7n survive 

 and intercross. Their offspring are again arranged as a^, 

 m^J and b^. But m^ stands at a higher level than in, 

 having been raised by the intermixture of m and a. 

 Similarly of succeeding generations. The average of the 

 species at 771^, '?7z^, mK tn^, and so on, is step by step raised 

 through the action of natural selection. Such a progressive 

 improvement of the species has been termed monotypic 

 evolution. So far there is no divergence. 



Divergence involves the segregation or isolation of the 

 diverging groups. The segregation may depend on geogra- 

 phical, ph3^siological, psychological, or other conditions ; but 

 there must be some means by which intercrossing and the 

 interblending of the divergent characters are prevented. 

 We may represent this roughly and diagrammatically 



thus : — 



g4 1^4 \^i 



I 



s2 m2 b2 



I 

 s^ m^ b^ 



s m b 



b* m' c* 

 b^ m^ c^ 

 b- an^ c^ 



I 



b m c 



Here the divergence of a strong variety (s, s"", etc.) from 

 a cunning variety (c, c^, etc.) is represented. Mediocritj'' 

 is again indicated by ?72, iii^, etc., and the individuals below 

 the average in these respects by 6, 6^, etc. These are 



