CHAPTER IX 



DIVERGENCE OF VARIATION 



1 T has long been known, though it has excited but little interest^ 

 that there is a great tendency in variation to be divergent. As 

 Guppy says (66, p. 104) Hooker, in his lecture upon Insular 

 Floras, "shadowed out a general notion of Centrifugal Variation 

 operating through countless ages'. It appears almost as a sug- 

 gestion, but the idea had been evidently floating half-formed in 

 his mind ever since he wrote his essay on the Tasmanian flora in 

 the late 'fifties. It was the nucleus of a theory of Divergence or 

 Differentiation that acquired more definite outlines as time went 

 on, since it reappears in the intensely interesting account of a talk 

 with Darwin which is given in a letter to Huxley in 1888 (19, 

 n, p. 306). 



" We can perhaps understand the long intervals of time now. 

 For the confirmation that such a theorv would have derived 

 from a line of research instituted on Darwin's lines was denied 

 to him. The two proved to be incompatible. For no inductive 

 process based on Darwin's lines could have found its goal in 

 a theory of centrifugal variation. 'I well remember', Hooker 

 describes in a letter to Huxlev in 1888, 'the worrv which that 

 tendency to divergence caused him (Darwin). I believe I first 

 pointed the defect out to him, at least I insisted from the first 

 on his entertaining a crude idea which held that variation was 

 a centrifugal force, whether it resulted in species or not.' Huxley 

 was in the same case. For he held views of the general differen- 

 tiation of types, and his road that would lead to the discovery 

 of the causes of evolution started from the Darwinian position. 

 That road was barred to him." 



There can be no doubt, when one looks at the various characters 

 that are used in taxonomic distinction between one form and 

 another, that the bulk of them are divergent, and that the more 

 so the higher one goes in the tables of characters, upwards from 

 species to families. Take for example the list of "family" 

 characters given in Appendix I, and note the great proportion of 

 distinctions in which there cannot even be an intermediate, by 

 reason of the marked divergence, and where, in any case, there 

 can be no functional difference between the intermediate and the 



