WEATHERWAX: THE EVOLUTION OF MAIZE 335 
flower; and the suppression of whole inflorescences and of vegeta- 
tive parts is also common. Whether this suppression is due to 
environment, or to inherent tendencies, or to a selection by the 
environment among the results of inherent tendencies, is unknown; 
but the comparative structure of Zea, Euchlaena, and Tripsacum 
indicates that, in their development from a common ancestor, 
certain tendencies have found expression in different ways in the 
different genera; and this expression has often come in the nature 
of a response to external conditions. In other words, we assume 
that these three genera are closely related, not because the char- 
acteristics which they have were present in a common ancestor, 
but because they have been evolved from an ancestor which had 
tendencies to suppress pistils in some flowers and stamens in 
others, whole flowers in some cases, and in some cases vegetative 
organs. Unless this assumption is made, we cannot base any 
close relationship between Euchlaena and Tripsacum upon monoe- 
cism, for the separation of the sexes in these two genera has been 
accomplished:in different ways. 
The first division of the progeny of this ancestral form set off 
a group in which the upper spikelet of each pair had lost its pistils, 
this giving rise to a number of genera of the Andropogoneae. On 
the other hand, in some of the plants whose inflorescences had 
retained their perfect flowers, the peduncles bearing groups of 
spikelets grew shorter (Fics. 15-19) until the spikelets were ar- 
ranged as in the tassels of maize or teosinte (FIGS. 3, 4, 5, 21). 
Pairs or larger groups of spikelets could have been left in this way. 
The plants in which the central spike of the inflorescence survived 
this process became the ancestors of Zea; while those in which 
this spike was lost gave rise to Euchlaena and Tripsacum. In the 
separation of these two genera, two lines of evolution were fol- 
lowed, depending upon the manner in which monoecism came 
about. 
Origin of Euchlaena.—In the line tending toward Euchlaena, the 
uppermost inflorescences—those terminating the main branches 
—lost their female elements and assumed the form of the present 
teosinte tassel (Fic. 20), while the lower inflorescences lost their 
male elements; between these two extremes were formed some 
mixed spikes, the upper portions being staminate and the lower 
