to which的疑惑，恳请各位朋友帮助！

Question

原句是：
Human facial expressions differ from those of animals in the degree to which they can be controlled on purpose.

在上面这句话里，to which的作用我不太清楚，很显然，这是一个定语从句，其分解成两句话便是：
Human facial expressions differ from those of animals in the degree.

They(human) can be controlled on purpose.

很奇怪的是，上面两句话中，没有了to，这是怎么回事？to which在此句话中应有的作用该是什么？请求赐教！谢谢！！！

Answer 1

to 在这句话里是修饰the degree的，换个方式说就是：Human facial expressions can be controlled on purpose to the degree. 就是说人类与动物面部表情的不同之处在于人类可以随意地控制面部表情的程度。

Answer 2

Human facial expressions differ from those of animals in the degree to which they can be controlled on purpose .
人类的面部表情不同于动物是两者在控制意识程度上存在差异。

同意"睡在屋顶的狗"的观点.关于前面的回答,differ from...本身就是一个短语,没有变化的.和...不同.
the degree to 是一个固定的短语,意思可以理解为"到...程度上,在某种程度上,...",而这里的in是来搭配它们的.就是说他们的区别存在于"in"在程度上的差异.in翻译为"存在于/在于"的意思.which代替的是degree,到某种程度上.

一个简单的例子是:
He differs from me in the degree to which we are remembered by people.他和我的不同可以体现在我们被人记住的程度上.

或者:
The way you manage your staff is reflected in how they treat customers and in the degree to which they are motivated and innovative in their approach.
你管理雇员的方式将从他们怎样对待客户，以及他们在工作中进取和创新的程度体现出来。

Answer 3

to 是跟differ...from...搭配的，

Answer 4

DIFFER TO`和简单的短语= =