394 E. C. ANDREWS. 
simply pinnate, without numerous leaflets, as the immediate 
ancestor of the genus, and one which gradually developed 
into the complex bipinnate form, On the one hand the 
genus is closely related to the tribe Kumimosez, and on 
the other hand to the tribe Ingez. The ancestral forms 
may be conceived as trees or shrubs of luxuriant type, 
flourishing in moist warm climates, and possessing leaves 
simply pinnate, persistent stipular spines, flowers in heads 
or spikes, corolla regular, and stamens definite and free. 
During the mild and genial climate of the later Cretaceous 
this type was transformed gradually in one direction to 
plants with indefinite monadelphous stamens and bipinnate 
leaves, namely, Ingez, in another direction to plants with 
definite free stamens and bipinnate leaves, namely, 
Humimosee, and in a third direction to plants with indefi- 
nite free stamens and bipinnate leaves, namely, Acaciez. 
The principal genera in these tribes are Pithecolobium, 
Mimosa, and Acacia, respectively. 
In Acacia, the section Gummifere is the type most 
uniform and most widely diffused, and from this fact, and 
the knowledge also that it is the form which exhibits the 
closest homology to the cognate genera, it is here con- 
sidered as the most primitive form of Acacia in existence. 
It may be noted that the members of the Gummiferee 
are lovers of open land and are xerophytes, even in the 
tropics. This feature obtains in America, Africa, Asia, 
and Australia alike, and indicates that the Acacia had 
developed a xeropbytic habit prior to the separation of the 
main tropical land masses from each other. If this period 
be considered as the late Cretaceous, it may be perceived 
that xerophytes among the dicotyledons had been estab- 
lished already in that period, either as a result of adaptation 
to the poor, sandy soil, or to a transient differentiation of 
climate, in the closing stages of the Cretaceous, or per- 
chance to a combination of these influences. 
