We come now to the second element of the whole process of evolution, namely, what we may call overproduction or excessive multiplication.  Like variation and so many other phenomena of nature, this is so real and natural that it escapes our attention until science places it before us in a new light.  The normal rate of reproduction in all species of animals is such that if it were unchecked, any kind of organism would cumber the earth or fill the sea in a relatively short time.  That this is universally true is apparent from any illustration that might be selected.  Let us take the case of a plant that lives for a single year, and that produces two seeds before it withers and dies; let us suppose that each of these seeds produces an adult plant which in its turn lives one year and forms two seeds.  If this process should continue without any interference, the twentieth generation after as many years would consist of more than one million descendants of the original two-seeded annual plant, provided only that each individual of the intervening years should live a normal life and should multiply at the natural rate.  But such a result as this is rendered impossible by the very nature which makes annual plants multiply in the way they do.  Let us take the case of a pair of birds which produce four young in each of four seasons.  Few would be prepared for the figures enumerating the offspring of a single pair of birds at the end of fifteen years, if again all individuals lived complete and normal lives:  at the end of the time specified there would be more than two thousand millions of descendants.  The English sparrow has been on this continent little more than fifty years; it has found the conditions in this country favorable because few natural enemies like those of its original home have been met, and as a consequence it has multiplied at an astounding rate so as to invade nearly all parts of North America, driving out many species of song birds before it.  About twenty years ago David Starr Jordan wrote that if the English sparrow continued to multiply at the natural rate of that time, in twenty years more there would be one sparrow to every square inch of the state of Indiana; but of course nature has seen to it that this result has not come about.  A single conger-eel may produce fifteen million eggs in a single season, and if this natural rate of increase were unchecked, the ocean would be filled solid with conger-eels in a few years.  Sometimes a single tapeworm, parasitic in the human body, will produce three hundred million embryos; the fact that this animal is relatively rare diverts our attention from the alarming fertility of the species and the excessive rate of its natural increase.  Perhaps the most amazing figures are those established by the students of bacteria and other micro-organisms.  Many kinds of these primitive creatures are known where the descendants of a single individual will number sixteen to seventeen millions