Why did the Shang Dynasty use tortoise shells and the Zhou Dynasty used yarrow for divination?

Yarrow and tortoise shell are ancient instruments for divination. Tortoise shell is the thick shell of tortoise. Turtles rely on this armor to avoid natural enemies. When it makes a slight movement, it immediately retracts its head, tail and limbs into the shell. In ancient China, the tortoise was once a symbol of good fortune, and was regarded as a sacred object to connect heaven and earth, and its status was quite respected. "Li Yun" records: "What are the four spirits?" Lin, phoenix, turtle and dragon are called the Four Spirits. "Turtles can be compared with legendary animals such as Kirin, Phoenix and Dragon, and their status is evident. In archaeological discoveries, jade turtles and other decorations have been found many times. It can be seen that in ancient times, people loved turtles. In people's eyes, the tortoise is sacred; Yarrow is a kind of plant similar to Artemisia, but it has 300 stems in 1000 years and its life span is long enough. It seems difficult to popularize such a divine plant. Sima Qian in Han Dynasty had a wide range of knowledge. At the age of twenty, he began to travel all over Kyushu, even in remote areas such as Bashu and Kunming at that time. However, when he wrote about yarrow in Historical Records, he could only hear the old man say that yarrow "has hundreds of stems, and was born here. There are no tigers and wolves among the animals, but there are poisonous thorns in the grass; "This statement seems to be close to real life, but it is estimated that there are no tigers, wolves and nontoxic yarrow to highlight the auspiciousness of yarrow.