根据所给中文完成句子：
In October 1838, I happened to read for amusement Malthus on Population, and being well prepared to _____ the struggle for existence which _____ goes on, from long continued _______ of the habits of animals and plants, it at once struck me that, under these circumstances, favorable variations would tend to be _______, and unfavourable ones to be destroyed.
[参考译文]1838年10月，我随便读了《马尔萨散人口论》，很欣赏书的到处都有的生存竞争，经过对动物和植物生存习性的长期观察，我忽然想到这种环境下，有利于生存的变化被保留了，而不利于生存变化被消除了。
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