314 PLANTS, SEEDS, AND CURRENTS 



climate, bird, and plant, the range of the bird being largely controlled 

 by the climate, and the range of the plant being mainly dependent 

 on the bird. In all cases there was the conception of a primal 

 world, where uniformity of conditions prevailed, and of primitive 

 generalised types that in their differentiation responded through the 

 ages to the diversification of their conditions. These views were 

 considerably extended in a paper on " Plant-Distribution from an 

 Old Standpoint," read before the Victoria Institute of London in 

 April 1907, when it was contended that the differentiation theory 

 presents us with a good working hypothesis for the age of the flower- 

 ing plants. They were further emphasised and enlarged in a paper 

 on " The Distribution of Plants and Animals " in Petermann's 

 Mitteilungen for 1910 (Heft II.). The paper was produced in a 

 German translation by one of his staff through the courtesy of the 

 editor ; and although printed in a somewhat abbreviated form there 

 was no impairment of the general line of the argument. 



But in all these statements of his views the author failed to recog- 

 nise that although the differentiation theory explained the diversity 

 of forms, there was much in distribution that it did not of itself 

 account for. What was lacking was the proper appreciation of the 

 part played by the arrangement of the continents in determining 

 during secular variations of climate plant- distribution. The de- 

 ficiency he has endeavoured here to supply; and whilst devoting 

 this chapter to the discussion of the differentiation theory, he will 

 deal with distribution as an expression of the geographical and 

 climatic conditions just mentioned in the chapter succeeding it. 



Statement of the Theory. — Natural families, as at present 

 recognised, seem to fall into two groups, the primitive and the 

 derivative, the first world-ranging and the second restricted in their 

 area. The primitive family as differentiation proceeds may give 

 rise to (a) zonal families, as in the case of the two closely related 

 families, the Primulacece of temperate latitudes and the Myrsinacece 

 of the tropical zone; (b) continental families, where they are 

 restricted to a continent, as in the cases of the Tropceolacece and 

 Sarraceniacece to America; and regional families that are confined 

 to a more circumscribed region, as in the case of the Goodeniacece to 

 Australia and its vicinity. 



That differentiation and decrease of range go together is a prin- 

 ciple that seems to prevail through the whole plant-world. It is 

 seen in its last stages in the role of the polymorphous or highly 

 variable species, which, whilst giving birth to varieties and local 

 races in different parts of its range, still covers most of the area of 

 the genus or sub-genus, as the case may be. It is seen even in the 

 behaviour of the variety so produced ; and thus the process goes on 

 until, as in Hawaii, different valleys and hill-tops may possess their 

 own peculiar forms. But this is a subject that has been already 

 discussed by the author in his work on the Pacific. Here he wishes 

 to emphasise the point that the behaviour of a polymorphous species 

 represents, though on a far smaller scale, the behaviour of the primi- 

 tive generalised family types that once ranged the globe. The 

 successive stages in the differentiation of a world-ranging family 



