the varieties are infinite; but two classes may be roughly seen.
鸟的飞行方式千差万别，但大体上可分为两类。
新概念英语第四册 Lesson 23 Bird flight

📚 句中词汇解析

从本句中提取的重要词汇

infinite ['ɪnfɪnət] 词频3 词根: in
adj. 极大的;无法衡量的; 无限的;无穷尽的; n. 无限的事物;无穷尽的事物; 上帝;
💡 in无+fin结束，范围+ite有...的 → 没有范围的 → 无限的
"The universe seems infinite."
宇宙似乎是无限的.
近义：infinity, boundlessness, illimitable, immeasurable, immense, incommensurate 反义：limited, qualified, constricted
roughly ['rʌfli] 词频3
adv. 粗略地;大体上;大致上;粗暴地;
"Red blood cells are roughly the shape of a disk."
红细胞大致呈圆盘状。
近义：about, approximately, around, loosely, nearly, barely 反义：minutely, painstakingly, scrupulously, thoroughly, carefully, circumstantially
seen [si:n] 词频1
v. 看见( see的过去分词 );观看;领会;考虑;
"The aforementioned person was seen acting suspiciously."
有人看见前面提到的那个人行动可疑。
varieties
n. 种种( variety的名词复数 ); 品种; 变化; （基于遗传差异的）变种
classes
n. 级别, 阶级（class复数形式）

📚 更多「新概念英语第四册」例句

The further off this solid obstruction, the longer time will elapse for the return of the echo.
固体障碍物越远。回声返回所用时间就越长。
It effectively spread the disease all over the continent and drastically reduced the rabbit population.
蚊子把这种疾病扩散到整个澳洲大陆，效果甚佳，结果兔子的数目大为减少。
like a vulgar flare amid the cool radiance of the stars,
如果人生不只是群星寒光中平凡的一闪，
The hovercraft is particularly useful in large areas with poor communications such as Africa or Australia;
气垫船在非洲、澳大利亚等交通不发达地区特别有用。
This decline in vigour with the passing of time is called ageing.
生命力随时间的流失而衰退叫做衰老。
drawing the particle first towards the advancing wave,
先把微粒拉向前移的海浪，