To the tune of “When Johnny comes marching home again,” the counting nursery-rhyme song starts out . . .

The animals went in two by two

Hoorah! Hoorah!

The animals went in two by two

Hoorah! Hoorah!

The animals went in two by two

The elephant and the kangaroo

And they all went into the ark

For to get out of the rain.

. . . and so on, with the number increasing in each stanza, three by three, four by four, etc., up to ten. A delightful version can be heard at https://www.youtube.com/watch?v=IuQjR2lsYl0 .

The assumption is that in order to repopulate the world after the Great Flood, you need at least two of each different kind of animal. There is a great biological truth in that, but the song ignores those animals that reproduce parthenogenetically, and consist only of females. In parthenogenesis (from the Greek for “virgin origin”), the females lay eggs and the eggs develop directly into more females, which lay more eggs, and so on – all without any intervention of males, of which in these species there are none.

The stanza above should therefore be the second one. I offer this as the proper first stanza:

The animals went in one by one

Hoorah! Hoorah!

The animals went in one by one

Hoorah! Hoorah!

The animals went in one by one

The gecko and aphid with never a son

And they all went into the ark

For to get out of the rain.