We have left for discussion only one topic dealing with heredity, but this is the most baffling of all, since it deals with the problem of how the various kinds of determiners came into existence. It is evident that if given one parent with all large-letter determiners and the other with all small-letter, we might, in the course of many generations, get a great variety of combinations and so a great many different-appearing individuals. But unless we have various kinds of determiners to start with, there is no way in which this can be done. We do not pretend to know very much about how the innumerable determiners that are in existence came about, but we have one clue that is thought to point the way. In some animals, and many plants, descendants put in their appearance from time to time that are so different from their ancestors as not to be accountable according to ordinary laws of heredity. These have long been known, and the name of “sport” has been applied to them by breeders. Since the facts about determiners have been learned, it has been clear that these “sports” cannot have all their determiners like those present in their parents, and it has come to be believed that occasionally spontaneous changes take place within individual determiners. Since the determiners are undoubtedly complex chemical structures, we know of no reason why this might not happen. Probably it is much more common an occurrence in some kinds of plants and animals than in others. The name of “mutant” has been applied to the plant or animal in which this has happened, and the process is called “mutation.” It is important since it is the most likely way in which the innumerable kinds of determiners that are now in existence came into being. We suppose that since life first put in its appearance on the earth there have been uncounted mutations, a vastly greater number than are now represented by determiners. Many of the mutants could not compete with their brothers and sisters of ordinary descent and so promptly died, but occasionally it might happen that a mutant would be as well fitted for life as its relatives, in which case it would establish itself, and in course of time become ancestor to a whole line like itself. If this happened often enough, and time were allowed for it to work out, all the kinds of plants and animals that are now in existence might have come by descent from a very few ancestors. The geological history of the earth shows that there has been plenty of time, even though valuable mutations did not occur oftener than once in a thousand years.