To illustrate this he undertakes a thought-experiment involving family photographs. If, hypothetically, there existed a complete set of photographs of all one's direct male ancestors arranged in order of birth date (or hatch date) from youngest to oldest stretching back millions of generations, from one generation to the next one would not perceive much difference between any two adjacent pictures—looking at a picture of one's grandfather or great-grandfather one is looking at a picture of a human—but if one looked at a picture from 185 million generations back one would see an image of some kind of fish-like animal. Dawkins stresses this point by saying the offspring of any sexually reproducing life form belongs in almost all cases to the same species as do its parents, with the exception of unviable hybrids such as mules.