quired the application of rather advanced concepts of 
plant breeding probably not at the disposal of pre- 
Columbian peoples in the New, or, for that matter, in 
the Old World. It may be that the deposition of lignin 
to form stone cells is related to more general physiological 
processes which were subject to selective influences. 
There is no apparent dissimilarity between early and 
later Phaseolus coccineus pods in this site. There are no 
pods which could be considered as intermediate in reduc- 
tion of hard texture and fiber between the early Tamau- 
lipas material and modern varieties. 
There are, then, remains of Phaseolus coccineus pods, 
anatomically more primitive than modern types, in in- 
habited sites and corresponding at later occupation levels 
(see introduction and Table IL) with definite evidences of 
agriculture. Several closely related problems remain to 
be solved. These are: the absence of P. coccineus from 
late Flacco to the beginning of Palmillas (Table II); the 
failure of the species to persist into historic times in this 
region; and its status as a domestic or wild plant during 
the periods of cave occupation. 
The disappearance of runner bean use in Tamaulipas 
resulted probably from the effect of climatic change on 
plant distribution. Preliminary studies (MacNeish, in 
conversation) indicate that this region has passed through 
at least two warm climatic cycles. The earliest of these 
was presumably warm and moist, as its duration was 
marked by the presence of Manihot from late Flacco to 
about the beginning of Palmillas (see Table IL). Runner 
bean remains are absent during this thermal period of 
more than 2000 years, although they are present both 
before and after. This absence cannot be explained by 
the prevalence of poor conditions for vegetal preservation 
in the caves, since cucurbit materials are abundant from 
this period (Whitaker, et al, 1957). 
[ 49 ] 
