But if, during the next month or so, she is heard to say "ball" on seeing a balloon, an Easter egg, a small round stone, and so on, doubts must begin to arise ; and it may then seem wiser to replace the first conclusion by the more cautious one that the word "ball" has now entered her vocabulary and that her knowledge of its meaning has begun to grow.
ポイント
replace A by (orwith) B 「AをBで置き換える」
接続詞 that 同格用法(~という):idea, thought, fact, hope, opinion のような「考え」、「意見」を示す名詞の後ろによく用いられる。
The idea that the earth is round was still unfamilier at that time. 「大地が丸いという考えは、当時はまだ知られていなかった」