Before illustrating these types, let us settle one or two matters of terminology. In the changes, the development and extinction, of species we must remember that such expressions as ‘a new species,’ or as ‘a species becoming extinct,’ are each commonly and indiscriminately used to express totally different and opposite meanings. Of course the ‘new’ species is not new in the sense that its ancestors appeared later on the globe’s surface than those of any old species tottering to extinction. Phylogenetically, each animal now living must necessarily trace its ancestral descent back through countless generations, through aeons of time, to the early stages of the appearance of life on the globe. All that we mean by a ‘new’ species is that from some cause, or set of causes, one of these ancestral stems slowly or suddenly develops into a form unlike any that has preceded it; so that while in one form of life the ancestral type is continuously repeated and the old species continues to exist, in another form of life there is a deviation from the ancestral type and a new species appears. Similarly, ‘extinction of species’ is a term which has two entirely different meanings. The type may become extinct by dying out and leaving no descendants. Or it may die out because, as the generations go by, there is change, slow or swift, until a new form is produced. Thus in one case the line of life comes to an end. In the other case it changes into something different. The huge titanothere, and the small three-toed horse, both existed at what may roughly be called the same period of the world’s history, back in the middle of the mammalian age. Both are extinct in the sense that each has completely disappeared and that nothing like either is to be found in the world to-day. But whereas all the individual titanotheres finally died out, leaving no descendants, a number of the three-toed horses did leave descendants, and these descendants, constantly changing as the ages went by, finally developed into the highly specialized one-toed horses, asses, and zebras of to-day. The analogy between the facts thus indicated and certain facts in the development of human societies is striking. A further analogy is supplied by a very curious tendency often visible in cases of intense and extreme specialization. When an animal form becomes highly specialized, the type at first, because of its specialization, triumphs over its allied rivals and its enemies, and attains a great development; until in many cases the specialization becomes so extreme that from some cause unknown to us, or at which we merely guess, it disappears. The new species which mark a new era commonly come from the less specialized types, the less distinctive, dominant, and striking types, of the preceding era.