All of the breeding Monarchs which make their way to eastern Canada each year have bread in the southern U.S.-the butterflies which reach Canada are one or two generations removed from those that overwintered in Mexico. 大概什么意思? 破折号后的内容 经过一两代后,到加拿大过冬的蝴蝶才能和那些去墨西哥过冬的蝴蝶真正区分开?