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
