INTROGRESSION AND EVOLUTION 67 



moved geographical barriers between previously isolated 

 species. On the other, he created new ecological niches in 

 which hybrid segregates might find a foothold. Some of 

 these niches were of definite types, and he created them 

 everywhere he went. Of these one of the most important was 

 his trash and dung heaps. He made these everywhere he 

 halted, and, as he unconsciously bred the quick-growing 

 weeds capable of utilizing soils high in nitrogen, he also un- 

 consciously carried them about from place to place and gave 

 them previously unparalleled opportunities to cross with 

 others of their kind and thus build up into superweeds. From 

 these weeds some of his crops were bred. There is good evi- 

 dence that hemp started in that way, and from what was 

 originally a weed plant there w^ere at length evolved hemp 

 as a fiber plant, hemp as a source of oil (from the seeds), and 

 hemp as a narcotic drug (Vavilov, 1926; Parodi, 1935). The 

 primitive chenopodiums and amaranths which are so widely 

 grown as cereals by primitive peoples, in both the old world 

 and new, show every indication of having originated in this 

 fashion. Many of the cucurbits probably originated in the 

 same way. Most, if not all, of the wild cucurbits are bitter 

 or insipid. Introgression produced weed types that became 

 camp followers. These were probably used first as dishes or 

 rattles. Increasing variation produced some whose seeds 

 were edible, and, still later, varieties with edible flesh were 

 selected. 



Evolution under domestication has been so complete that 

 it is difficult to get exact data on the subject. In only a few 

 instances can we point to the exact wild species from which a 

 cultivated plant or a weed was derived. For some of the 

 cultivated plants we know closely related wild species, 

 though we have little or no evidence of the exact relation 

 between them and the cultivated plant. In many other 

 cases we can point to a group of weeds that are related to a 

 cultivated plant. This is no solution to the problem. We 

 now know that weeds may be bred from cultivated plants, 

 as well as vice versa. Since weeds as we know them are 



