Go into Photoshop, Illustrator, or hell, even Paint. Any image editing program where you can select a color from a color spectrum. If you take a look at this spectrum, you'll see that the colors blend together seamlessly. You have a red, then another red that's almost but not quite identical to the first red, then another red that's almost but not quite identical to the second red, and so on. Eventually, you'll notice the red start to become slightly orange. Keep going, and it will be just as orange as it is red. Keep going even more, and you'll see it start to become lighter. At some, not even the slightest hint of red is visible, and you have yellow. Evolution works in a similar way. Each generation is extremely similar to the previous, with just a slight change. A fish won't just become a lizard, just like red won't just become yellow. The fish begins to resemble an amphibian, then it becomes more amphibian than fish, then becomes more reptilian than amphibian.