20.If it were not for this faculty,they would devour all the food available in short time and wouldprobably starve themselves out of existence.
(虚拟语气)
如果不是具有这种能力,它们就会在短时间内吞掉所有能得到的食物,而且很可能会因饿死而灭绝。
分句1: Ifit were not for this faculty
分句2: they would devour all the food available in short time
分句3: would probably starve themselves out of existence
分句1 是虚拟句的条件从句
分句2、3是并列, 主句