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