bling the ear of maize. These steps, not necessarily in 
this order, are as follows: 
1. The solitary pistillate spikelets characteristic of teo- 
sinte become paired spikelets typical of maize as the 
aborted member of each pair becomes functional. 
2. The axis of the rachis shortens with the result that 
adjacent rachis segments assume positions in the same 
horizontal plane and become ‘‘yoked”’ in pairs. This 
change, combined with the preceding one, results in a 
four-rowed ear. 
3. A twisting of the axis causes alternate segments to 
assume positions in a plane at right angles to that occu- 
pied by adjacent segments above and below. This, com- 
bined with the changes already described, produces an 
eight-rowed ear. Ears with higher row numbers are pro- 
duced by further twisting of the axis. 
There is no doubt that the series of transition forms 
which Collins described do occur in segregates of maize- 
teosinte hybrids. Nor is there any doubt that the ear of 
maize could have arisen from the spike of teosinte through 
such a series of changes. Indeed if maize had originated 
from teosinte the changes which have actually occurred 
must have been very similar to those which Collins has 
described. 
Collins did not imply, however, that maize had actu- 
ally been derived from teosinte by this series of steps and 
in fact he carefully pointed out that these intermediate 
steps explained the evolution of the ear only in a me- 
chanical sense. 
Even in a strictly mechanical sense, however, the hy- 
pothesis has not been satisfactory. Weatherwax (19) has 
objected to it on the grounds that there is no evidence 
in maize of the supposed ‘‘yoking’’ of pairs of spikelets 
on opposite sides of the rachis. Collins believed that he 
had evidence for yoking in ears in which some of the rows 
[ 44 ] 
