DEVELOPMENT AND DISTRIBUTION OF LEGUMINOSAE. 367 
small patches of the fertile tropics in Australia, just as 
though they had been gathered from the world-wide tropics 
and carefully guarded from marked modification, while 
Hucalyptus and the phyllodineous Acacias have overrun 
Australia both in species and individuals. Nevertheless, 
outside Australia the Kucalypt and the leafless Acacia only 
occur as waifs or strays which have undergone but slight 
modification. 
To understand the development of a genus the factors of 
evolutions must be considered. -These comprise selection, 
heredity, environment, and variations. But traced back- 
ward far enough, geographical environment appears to be 
the key to evolution. 
One genus may be endemic and yet appear to be ancient, 
another may be cosmopolitan and yet appear tobe recent and 
derivative. Such a conclusion needs careful consideration 
since the operation of one principle must not be permitted 
to clash with that of another. A study of the next chapter 
will help the student in this connection. 
For example, the HKugenias, as classified by Bentham, 
the Myrtles, Erythrinas, Sophoras, and Dalbergias, of 
Australia, flourish in the fertile tropical or subtropical 
forests or jungles; they present great similarities in general 
appearance to these genera in Asia, and other places, so 
much so that they appear recent and derivative. The 
Hucalypts, Melaleucas, the phyllodineous Acacias, the 
Pultenzeas, Dillwynias, Daviesias, and Jacksonias, on the 
other hand are peculiar and have no close relations in other 
portions of the globe. The luxuriant types by their rich 
colouring and their delicate leaves give the impression of 
youth for these types, while the rusty, dilapidated, weather- 
beaten, tough, and stunted appearance of the majority of 
the endemic genera cited give the impression of great 
age. 
