The idea of variation was based upon simple observation. Dr. Herbert Walter has said that “variation is the most constant thing in nature,” and paradoxical as that may seem, it is nevertheless true. No man looks exactly like another man, no tree exactly like another tree, no shell exactly like another shell. The Japanese artists appreciate this variation, and make use of their knowledge in painting, which is one of the reasons why their art is not readily appreciated by the occidental who is much inclined to “lump” things. No Japanese artist would think of painting two dogs, or two streams, or two houses that resembled each other in every respect, for he knows that every thing in the universe, whether it be alive or dead, organic or inorganic, differs from every other thing in the universe. Sometimes the difference is easily seen, as that between a shark and a goldfish, or a Negro and a Scandinavian or Teuton. At others it is almost indistinguishable, and can be discovered only by the most accurate micrometer, or the most precise chemical analysis. But always the difference exists, the variation is present, and this fact is the basis for Darwin’s belief in the inborn necessity for all living things to vary. The second factor, that of a struggle for existence, was suggested to Darwin by a reading of Malthus’ classic paper on population. All creatures normally tend to increase in numbers. Mating fish produce millions of eggs in a season; chickens rear nestfulls of young; rabbits and guinea-pigs produce litter after litter of young from the matings of two parents--everywhere, both in nature and in domestication, living things seem to be on the increase. And yet we have no evidence that (excluding the rather doubtful influence of man) there are more animals on earth today than there were half a million years ago; the probabilities are that there are fewer. Clearly, therefore, some process is at work which prevents the seeming increase from taking place. In order to understand something of the complexity of this process, let us select a specific example. Among marine animals, the oysters are remarkable for the immense numbers of eggs which they produce--the average for the American oyster is probably about 16,000,000. If all the progeny of a single oyster were to live and reproduce, and their progeny were to do likewise, and so on until there were great-great-grandchildren, the total number of oysters that were descendants of the original pair would be about 66,000,000,000,000,000,000,000,000,000,000,000 and their shells would make a mass eight times as great as the earth.