FROM AN OLD STANDPOINT. 



175 



localised groups (generic or otherwise) have been developed. 

 The same process may be seen in operation within the limits of 

 a genus. ^lany tropical genera, as I have shown in my book 

 on Plant-Dispcrsol, possess in addition to tlie more localised 

 species, a highly variable species that occupies the range of the 

 genus, and establishes centres of difierentiation all over the 

 area. 



It can scarcely be doubted that if we Ijegin by postulating a 

 world-ranging, generalised type, which in the course of ages 

 differentiates in sitv. into tribes, genera, and species, we should 

 be spared a sea of trouble in the investigation of plant-distri- 

 bution. All the ditticulties of disconnected distribution would 

 disappear. Many botanists must have at times felt the need 

 of an hypothesis of this kind, though few would be prepared 

 to abandon the old position. Amongst those who in recent years 

 have revolted against the haljit (to use the words of Sir William 

 Dawson) of laboriously devising expedients for the migration of 

 plants and animals is Dr. Karl Mueller.* With the case of 

 Liriodendron in his mind, he observers that all explanations of 

 origin by migrations and bridges cease, and we are forced back 

 on the idea of autochthonous causes." So, again, the occurrence 

 of a species of Baobab (Adansonia) in Australia and South Africa 

 causes him to remark that '•' the enigma cannot be explained by 

 migration ; the same conditions of creation produced in dilierent 

 places the same type, only in different species." 



Difficulties connected with Families. 



But apart from questions connected with genera, many 

 difficult problems concerning families appear much less 

 formidable when we regard them from the standpoint of the 

 differentiation theory. There is the matter of large and small 

 families. Take, for instance, a great family like the Araceie, 

 distributed all round the globe, and possessing a multitude of 

 genera. Then take a very small family like the Columelliaceie, 

 containing only one genus, limited to Ecuador and Peru. The 

 family characters of the Aracea:- are those of the undiff'eren- 

 tiated primitive type. Where, we may ask, are the primiti\'e 

 family characters of the Columelliaceie ? They exist, but only 

 as expressions of a simple family type, the genetic connections 



^ See Trans, and Proc. Xe'.r Zealand Institute, xxv, 1892, for a trans- 

 lation by H. Suter of Dr. Mueller's paper in Das Ausland, Julv 20th, 

 1891. 



