SVENSON: TAXONOMY IN RELATION TO GEOGRAPHY 47 



Course of Evolution by Differentiation or Divergent Mutation," is a negation 

 of evolution by the natural selection of variations as propounded by Darwin 

 and Wallace and affirmed by Asa Gray. The idea is not an original one, but is 

 based largely on what Guppy called "differentiation," in his work on the vege- 

 tation of tropical islands. The eleventh chapter leads out, "Natural selection, 

 being a common phenomenon of everyday experience, has exercised such a fas- 

 cination that it has to a notable extent inhibited people from trying properly to 

 think out how a principle, whose essence is competition with partial escapes 

 into usually temporary success every now and then by improved adaptation, can 

 produce the ordered arrangement, taxonomy, and morphological or structural 

 uniformity with which we are familiar" (p. 103). Many, if not most or even 

 all, of the characters of distinction that mark families, sub-families, and even 

 smaller groups, are such that they can have no serious value upon the physio- 

 logical side which is the only one that matters from the point of view of natural 

 selection or gradual adaptation. These mutations are assumed by Willis to re- 

 quire long periods of time and to occur infrequently. "If one suppose a genus to 

 give off new species more or less in proportion to the area that it covers (which 

 again will be more or less in proportion to its age among its peers), it is clear 

 that all the offspring will carry a large proportion of the characters of the par- 

 ent, and that therefore while offspring arising near together will be most likely 

 closely to resemble one another, there is no reason wjiy a close resemblance 

 should not arise with a wide geographical separation" (pp. 155, 156). "It will 

 commonly be found, in studying the distribution of the species of a genus, es- 

 pecially if it be of small or moderate size, that they are more densely congre- 

 gated toward the centre of the distribution of the genus, and fall off gradually 

 toward the edges, so that when one draws a line round the outermost localities 

 of each species one obtains a picture not unlike that which is called a contour 

 map by geographers ..." thus, Willis illustrates the distribution of the species 

 of Ranunculus found in New Zealand (pp. 149, 150). "Here one finds 'wides' 

 (as I have called the species which have a dispersal outside the country in ques- 

 tion) occupying the whole area of the islands of New Zealand, and also reach- 

 ing eastwards to the Chatham Islands, 375 miles away . . . The endemics are 

 evidently crowded together rather south of the middle of the South Island, 

 whilst they fade out completely before the north end of the North Island is 

 reached . . . The general impression that one gains from a map like this is that 

 the genus Ranunculus entered New Zealand probably from the south, and at 

 some place in the southern half of the South Island, where the incoming species 

 began giving rise to endemics, and on the average each species, wide or en- 

 demic, spread to the distance allowed by its age, and suitability to the conditions 

 with which it met." 



