Then the LORD said to Noah, “Go into the ark, you and all your household, for I have seen that you are righteous before me in this generation. Take with you seven pairs of all clean animals, the male and his mate, and a pair of the animals that are not clean, the male and his mate, and seven pairs of the birds of the heavens also, male and female, to keep their offspring alive on the face of all the earth. For in seven days I will send rain on the earth forty days and forty nights, and every living thing that I have made I will blot out from the face of the ground.”
And Noah did all that the LORD had commanded him.
– Genesis 7:1-5
In these verses are contained the instructions for the preservation of the animals in the Ark. A male and a female of each “kind” were to be brought into the Ark, “to keep them alive.” The scope was quite comprehensive: “two of every sort.” God had a purpose for each created kind, so He intended that all species be preserved through the Flood. In addition to this general rule, seven animals of each “clean” kind were to be taken on board (7:2). Evidently those “clean” were intended for use as domestic and sacrificial animals.
Most land animals are small, of course; so this did not by any means represent an impossible task. Authorities on biological taxonomy estimate that there are less than eighteen thousand species of mammals, birds, reptiles, and amphibians living in the world today. This number might be doubled to allow for known extinct land animals. Allowing then for two of each species, there might have to be a total of about seventy-two thousand animals on the Ark—say seventy-five thousand; to allow for the five extra animals in each “clean” species.
The Ark was of immense size! It could have carried as many as one hundred and twenty-five thousand sheep, and since the average size of land animals is surely less than that of a sheep, it is obvious that no more than 60 percent of its capacity would have to be used for animals.
There were a few large animals (elephants, dinosaurs, giraffes, etc.) to be carried on the Ark, but many more small ones (mice, birds, lizards, frogs, etc.). Even the large animals were probably represented by young (therefore small) individuals, since they had to spend a year in the Ark without reproductive activity and then go out to repopulate the earth.
Thus, the specified size of the Ark seems ideally appropriate for the animals it had to carry. There was ample room for food for the animals, for living quarters for Noah and his family, and for any other necessary purposes.