In the very distant geological past, all animals were aquatic. The very first vertebrates or animals with backbones, of which we have any fossil record, lived in water. These vertebrates, the fish, were adapted to underwater living. Their streamlined bodies were covered with scales to reduce surface friction: they had muscular tails so that they could swim swiftly in such a dense medium as water; and they were endowed with gills for breathing underwater. Descendants of fish-type ancestors crossed the seashore barrier and accommodated themselves to life on land. As amphibians, they possessed limbs instead of fins and lungs instead of gills. But they never became completely free of the bonds that tied them to the water; even today many amphibians return to the water to lay their eggs. Millions of years after the first clumsy amphibians crawled over the land, newer types of land dwellers appeared, these animals give rise to the present-day reptiles and mammals. They were completely converted for land dwelling, with bodies and biological activities far different from those of fish. With these special adaptations, mammals have been able to colonize the woods and meadows, the deserts and high mountains, often far removed from the sea.

The seashore was a barrier for descendants of fish-type because………

Câu hỏi liên quan

• Which of the following is NOT mentioned about John Harvard?

• All of the following were mentioned in the passage as detriments to women in the business world EXCEPT .................

• Normal human beings ...........

ZUNIA12

• It can be inferred from the passage that John Adams was: 

• A typical African woman spends _______ collecting firewood every day.

• The word “elaborate” in paragraph 3 is closest in meaning to ...........

• The word “oscillating“ in line 17 is closest in meaning to ........

• The word “minute” in the second paragraph is closest in meaning to _______.

• What percent of the earth’s area do rain forests cover?

• The phrase "held back” in paragraph 1 means________.

• According to the passage, the doers of education are .............

• This article is about ________.

• The author implies that the mammals of the "suburban forest" differ from most species of trees there in which of the following ways?

• What does the word “filters out” in paragraph 1 probably mean?

• Facial expressions __________.

• How did the publication of Moby Dick affect Melville’s popularity?

• All of the following are defined in the passage EXCEPT ............

• The word “basis” in paragraph 1 is closest in meaning to_________.

• Where did Moore spend most of her adult life?

• According to the passage, how are ants guided by trail pheromones?