PLA.ST DISTIIIBITION ll3 



ancient family type successively into tribes, genera, and species, and 

 that the method adopted by many a monographer in dealing with a 

 family is the method that has been followed in nature in this respect. 

 On the origin of the family types such conclusions could have no 

 bearing. 



Postulating the original existence of world-ranging generalized 

 family types during an era of uniform conditions, it is argued that the 

 differentiation of these primitive types was in response to the ])ro- 

 gressive differentiation of their conditions. Allusion is then made to 

 the dilemma in which all theorists find themselves when they come to 

 handle the larger groups, a dilemma where they assvime the original 

 high mutabiliry of characters that have been through the ages rela- 

 tively fixed and iunnutable. If stable now, why so unstable then r" 

 The dittlculty has to be faced ; and we are led to believe that the age 

 that witnessed the rise of the great families was an age of mutations 

 free and unchecked, the mutability decreasing and the original modi- 

 fi^'ations becoming more and more fixed with the progressive diffe- 

 rentiation of conditions. In other words, mutahilify is the watchword 

 for the pre-differentiation era and adaptioity for the era that followed. 

 Prominence is given in these connections to the work of Dr. Willis 

 on the Podostemacese, and the writer's position with regard to his 

 " Age and Area " hypothesis is defined. 



The distribution of families is then treated statisticallv, and it is 

 shown that whilst they largely ignore the cleavage of the land into 

 two great masses, diverging from the north, they respond in marked 

 degree to the differentiation of the climatic zones. Behind their 

 disregard of the existing arrangement of continents and oceans lies the 

 story of the first era, and behind their ready response to climatic 

 diff'erentiation lies the story of the second era. In the circumstance 

 that the response to the bieleavage of the land-mass is either absent 

 or small in the larger groups and becomes greater and greater as we 

 descend the differentiating scale, until it attains its maximum in the 

 species, is recognized the contrast between the pre-differentiation era 

 and the era Avhen diff'erentiation reigned supreme. It is held that 

 there is a method here disclosed that could only arise through the 

 family differentiating into tribes, the tribes into genera, and the 

 genera into species, since the opposite plan of beginning with the 



species would produce chaos With regard to the application of 



similar views to the distribution of animals, it is contended that the 

 fundamental difference in the schemes of development of plants and 

 animals ought to be reflected in the different application of the 

 doctrine of evolution. 



The paper ends with the statistical treatment of the larger grou])s 

 behind the families ; and it is shown that whilst the Dicotyledons 

 disjjlay a much greater tendency to detachment from the tro])ics than 

 do the Monocotyledons, the Sympetake stand foremost in this respect 

 amongst all the primaiy groups. 



JuLI!>AL Ul' iJuTAM. — \'uL. oG. lAl'lJlL, lUlS."! 



