The argument is plausible; and in order to understand its fallacy we must give our attention to a fact, not generally realised, which is involved in all practical reasoning about all causes whatsoever.
If we use the word "cause" in its strict speculative sense, the number of causes involved in the simplest effect is infinite.

Let us take, for example, the speed of a horse which wins a race.

Why does the speed of this horse exceed that of the others?
We may in answer point to qualities of its individual organism.

But these will carry us back to all its recorded ancestors--sires and dams for a large number of generations: and even so we shall have been taken but a small part of our way.

The remotest of these ancestors--why were they horses at all?
For our answer we must travel through the stages of organic evolution, till we reach the point at which animal and vegetable life were one.