When Charles Darwin explained evolution (进化), the process he saw was natural selection (选择). It turns out human selection can also cause plants to evolve. Take the case of the snow lot us, a rare (稀小的) plant that grows only high up in the Himalayas. Researchers have discovered that type of the plant has been becoming smaller over time—the one people like to pick. A type of snow lotus called Saussurea Laniceps is used in Tibetan and Chinese medicine and is increasingly demanded by tourists. The largest plants are picked, and that occurs during their only flowering period. The result is that only smaller, unpicked plants go to seed. Selection caused by human beings is a powerful force, according to some scientists. They studied the plants picked over time and also compared the snow lotuses growing in heavily harvested areas with those found in places where people rarely picked them. They reported in an article that the heavily harvested snow lotuses have been getting shorter over the last century, while those in areas not picked have remained the same. They found that in the heavily harvested areas, where only the smaller plants got to reproduce, the average height was 5.3 inches. The same plants in rarely harvested areas averaged 9 inches in height. And they noted that another type of snow lotuses which is not often picked, has not been changing. What does the word 'that'(Line2, Para.3) refer to?