根據(jù)所給中文完成句子：
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.
[參考譯文]
1838年10月，我隨便讀了《馬爾薩散人口論》，很欣賞書的到處都有的生存競(jìng)爭(zhēng)，經(jīng)過對(duì)動(dòng)物和植物生存習(xí)性的長(zhǎng)期觀察，我忽然想到這種環(huán)境下，有利于生存的變化被保留了，而不利于生存變化被消除了。
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.
[參考譯文]
1838年10月，我隨便讀了《馬爾薩散人口論》，很欣賞書的到處都有的生存競(jìng)爭(zhēng)，經(jīng)過對(duì)動(dòng)物和植物生存習(xí)性的長(zhǎng)期觀察，我忽然想到這種環(huán)境下，有利于生存的變化被保留了，而不利于生存變化被消除了。