Because it is still a little bird too. maybe it feel afeard in its heart when it faces to the biger.

但是对于一些大点的动物，如野兔，还是有点困难。

The sapropel-type organic matter evolution need higher temperature and biger depth and its main gas generating zone is deeper than humic-type organic matter's.

腐泥型有机质的演化需要更高的活化能，而需更高的温度和更大的深度，其生气主带深于腐殖型有机质源岩层的位置。