decline是指 力量、数量的减少. descend是指在空间上的下降.
descent
英 [dɪ'sent] 美 [dɪ'sɛnt]
n. 下降；血统；袭击
vt. 除去…的气味；使…失去香味
例句：
Americans of Chinese descent. 中国血统的美国人
The authors show how this was triggered by his belief in the common descent of all humans.
作者表明，这张图是如何由他的信仰引发出来的，达尔文相信全人类属于共同的血统。
descend
英 [dɪ'send] 美 [dɪ'sɛnd]
vi. 下降；下去；下来；遗传；屈尊
vt. 下去；沿…向下
例句：
1.Should you reach the summit you would have only one desire, and that to descend and be with those who dwell in the deepest valley.
一旦你登上顶峰，你就只有一个愿望，那就是往下走入最深的峪谷里，和那里的人民一同生活。
2.The bell pulses to give some directional control, but mostly they drift with the wind. It expels gasto descend, and expels water from trim bladders to rise.
膜囊有规律的摆动以提供一些方向性的控制 但它们主要是随波逐流.它通过排出气体来下降 通过压缩气囊排出水来上升。