。
Birds of a feather flock together.
Literal
Kind [by-をもって] gather.
A classical proverb, 類をもって集まる (also 類は友を呼ぶ). をもって ('by, with, by means of') is a classical/formal way to mark instrument or means — in modern Japanese mostly preserved in set phrases and formal speech. 類 ('kind, type') refers here to 'like with like.'