Darwin believed that the little variations we see between one person and another, one fish and another, when chosen by natural selection, could build up in an unlimited way. You could breed guppies, selecting for large tails. By allowing only the guppies with the largest tails to breed you are able to get guppies with tails more than twice as big as the ones started with. This would correspond in nature, but because of genetic limits it is impossible to develop either a breed of guppies with tails as big as a whale, or develop whales from guppies by selective breeding. Genetic limits are real! They are real for natural selection as well as for selective breeding. Changing works to a point, but don't do like John the book keeper who was good at math and knew that his son John Junior was five feet tall and was now growing about an inch a year. Junior had outgrown his bed, and John wanted to get him one he could use all the rest of his life, so John did the arithmetic. Figuring the inch a year on out into the future, if Junior lived another 60 years it should add another 60 inches, he should be 10 feet tall! So John special ordered a bed just over ten feet long. He did not take into account a genetic limit that permits growth for a period, and then stops it. Please Register or Log in to view the hidden image!