384. This is why the number of signals that an animal can make is very limited: the great tit(山雀)is a case in point; it has about twenty different calls, whereas in human language the number of possible utterances is limitless.

英语四级译文:
这就是动物所能发出的信号数量极为有限的原因：山雀就是个典型的例子，它有大概20种不同的叫声，而在人类语言中可能做出的信号数量是无限的。

四级词汇讲解:
this is why...意为“这就是...的原因”。如:
This is why he didn't come.这就是他没来的原因。

a case in point意为“与所谈的事有关的例证，事例”，。如:
As a case in point, let's look at nineteenth-century France.作为典型的例子，我们来看看19世纪的法国。

whereas此处意为“然而，而”。如:
She earns $10,000 a year whereas he gets at least $100,000.她一年挣1万美元，而他至少可以挣到10万美元。

英语四级考点归纳:
this is why...具体说明的是结果，而想要由结果导出原因，应该用the reason why...。如:
The reason why I'm late is that I missed the bus.我迟到的原因是我没赶上公共汽车。
The reason why she didn't come to school is because she got ill.她没来上学的原因是她生病了。