DIFFERENTIATION 



319 



indebtedness to Mr. R. H. Cambage. Like the Myrtacece, the 

 Leguminosce are regarded as descended from a few uniform primary 

 types widely diffused through the world in Cretaceous times and 

 differentiating in later geological ages in response to the differentia- 

 tion of conditions. We have here in geological time the same asso- 

 ciated processes of differentiation of conditions and of differentiation 

 of types. The Upper Cretaceous period, when the primary type 

 had a wide range over the globe, was characterised by low-lying 

 lands and by a mild, moist and genial climate extending from 

 the tropics to the polar regions. Progressive differentiation of 

 climate in later geological times, when high mountains, large con- 

 tinents, and great deserts came into existence, found a response in 

 the differentiation of the types, many of them responding in the 

 present temperate latitudes to the changes in their environment by 

 the development of large and important groups of xerophytes, the 

 evolution taking place along divergent lines in different regions. 



In discussing the principles of distribution he lays stress on the 

 probable great age in Australia of the " pantropical " genera and 

 on the relative youth of the endemic genera. It is shown that in 

 Australia the plants which respond to the xerophytic conditions 

 that prevail there, such as the Eucalypti and the phyllodineous 

 Acacia?, have only in recent geological times assumed their present 

 leaf-forms. They represent Australia's response in the differentia- 

 tion of plant-types that were originally widely spread in the tropics ; 

 and the more recent development of such generic forms in compari- 

 son with their parent pantropical types illustrates a principle that 

 has long been recognised as a corollary of the theory of differentia- 

 tion. If the theory is true, this is its natural consequence ; and the 

 principle involved, namely, that antiquity does not connote change, 

 has been already discussed in an earlier page of this chapter. 



Mr. Andrews returns to the attack on those who would deny to 

 Australia the right to the sole possession of Eucalyptus. " The 

 evidence " (he says) " is overwhelming against the probability of any 

 dicotyledonous genus which is endemic in Australasia having existed 

 in any other continent in either Cretaceous or Tertiary time." He 

 maintains, and this is a strong point, that if xerophytic types, like 

 those of Eucalyptus, the phyllodineous Acacias, Banksia, etc., had 

 gained access to the waste areas of the other southern land-masses, 

 and particularly South Africa, they would have found a congenial 

 home. His general position as regards Australia may be thus 

 summed up. He would hold that whilst the endemic vegetation of 

 Australia has been developed within its limits as the result of its 

 special conditions, the source of its affinities with South Africa and 

 South America must as a rule be looked for in the common home 

 of the type in the tropics. Differentiation of types in response to 

 differentiation of conditions is evidently, to use my own language, 

 the bed-rock of the views advocated by Mr. Andrews. To employ 

 his own words : " Traced backward far enough, geographical environ- 

 ment appears to be the key to evolution." 



The Natural Order and Darwinian Evolution. — It follows 

 from the foregoing remarks that no plant-groups, in the sense of the 



