complete knowledge stands first; but second only stands the research which, while enabling us clearly to pose the problem, also requires us to say that with our present knowledge we can offer no complete solution.

Let me illustrate what I mean by an instance or two taken from one of the most fascinating branches of world-history, the history of the higher forms of life, of mammalian life, on this globe.

Geologists and astronomers are not agreed as to the length of time necessary for the changes that have taken place. At any rate, many hundreds of thousands of years, some millions of years, have passed by since in the eocene, at the beginning of the tertiary period, we find the traces of an abundant, varied, and highly developed mammalian life on the land masses out of which have grown the continents as we see them to-day. The ages swept by, until, with the advent of man substantially in the physical shape in which we now know him, we also find a mammalian fauna not essentially different in kind, though widely differing in distribution, from that of the present day. Throughout this immense period form succeeds form, type succeeds type, in obedience to laws of evolution, of progress and retrogression, of development and death, which we as yet understand only in the most imperfect manner. As knowledge increases our wisdom is often

turned into foolishness, and many of the phenomena of evolution which seemed clearly explicable to the learned master of science who founded these lectures, to us nowadays seem far less satisfactorily explained. The scientific men of most note now differ widely in their estimates of the relative parts played in evolution by natural selection, by mutation, by the inheritance of acquired characteristics; and we study their writings with a growing impression that there are forces at work which our blinded eyes wholly fail to apprehend; and where this is the case the part of wisdom is to say that we believe we have such and such partial explanations, but that we are not warranted in saying that we have the whole explanation. In tracing the history of the development of faunal life during this period, the age of mammals, there are some facts which are clearly established, some great and sweeping changes for which we can with certainty ascribe reasons. There are other facts as to which we grope in the dark, and vast changes, vast catastrophes, of which we can give no adequate explanation.

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 indiscrimi

nately 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 eons 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.

When dealing with the changes, cataclysmic or gradual, which divide one period of paleontological history from another, we can sometimes assign causes, and again we can not even guess at them. In the case of single species, or of faunas

of very restricted localities, the explanation is often self-evident. A comparatively slight change in the amount of moisture in the climate, with the attendant change in vegetation, might readily mean the destruction of a group of huge herbivores with a bodily size such that they needed a vast quantity of food, and with teeth so weak or so peculiar that but one or two kinds of plants could furnish this food. Again, we now know that the most deadly foes of the higher forms of life are various lower forms of life, such as insects, or microscopic creatures conveyed into the blood by insects. There are districts in South America where many large animals, wild and domestic, can not live because of the presence either of certain ticks or of certain baleful flies. In Africa there is a terrible genus of poison fly, each species acting as the host of microscopic creatures which are deadly to certain of the higher vertebrates. One of these species, though harmless to man, is fatal to all domestic animals, and this although harmless to the closely related wild kinsfolk of these animals. Another is fatal to man himself, being the cause of the "sleeping-sickness" which in many large districts has killed out the entire population. Of course the development or the extension of the range of any such insects, and any one of many other causes which we see actually at work around us, would readily