Chapter 18

There is reason, too, for winged creatures having feet, but fish none. The former have their home in the dry medium, and cannot remain always in mid¹ air; they must therefore have feet. Fish on the contrary live in the wet medium, and take in water, not air. Fins² are useful for swimming, but feet not. And if they had both they would be non-sanguineous. There is a broad similarity between birds and fishes in the organs of locomotion³. Birds have their wings on the superior part, similarly fish have two pectoral fins; again, birds have legs on their under parts and near the wings; similarly, most fish have two fins on the under parts and near the pectorals. Birds, too, have a tail and fish a tail-fin.

1 mid   

adj.中央的，中间的

参考例句：

• Our mid-term exam is pending.我们就要期中考试了。
• He switched over to teaching in mid-career.他在而立之年转入教学工作。

2 fins   

[医]散热片；鱼鳍；飞边；鸭掌

参考例句：

• The level of TNF-α positively correlated with BMI,FPG,HbA1C,TG,FINS and IRI,but not with SBP and DBP. TNF-α水平与BMI、FPG、HbA1C、TG、FINS和IRI呈显著正相关，与SBP、DBP无相关。 来自互联网
• Fins are a feature specific to fish. 鱼鳍是鱼类特有的特征。 来自辞典例句

3 locomotion   

n.运动，移动

参考例句：

• By land,air or sea,birds are masters of locomotion.无论是通过陆地，飞越空中还是穿过海洋，鸟应算是运动能手了。
• Food sources also elicit oriented locomotion and recognition behavior patterns in most insects.食物源也引诱大多数昆虫定向迁移和识别行为。