i 7 4 THE POPULAR SCIENCE MONTHLY. 



they are found are sedimentary deposits ; and each of these proposi- 

 tions is founded upon the same axiom that like effects imply like 

 causes. If there is any cause competent to produce a fossil stem, or 

 shell, or bone, except a living being, then paleontology has no foun- 

 dation ; if the stratification of the rocks is not the effect of such causes 

 as at present produce stratification, we have no means of judging 

 of the duration of past time, or of the order in which the forms of 

 life have succeeded one another. But, if these two propositions are 

 granted, there is no escape, as it appears to me, from three very im- 

 portant conclusions. The first is, that living matter has existed upon 

 the earth for a vast length of time, certainly for millions of years. The 

 second is that, during this lapse of time, the forms of living matter 

 have undergone repeated changes, the effect of which has been that 

 the animal and vegetable population at any period of the earth's his- 

 tory contains some species which did not exist at some antecedent 

 period, and others which ceased to exist at some subsequent period. 

 The third is that, in the case of many groups of mammals and some of 

 reptiles, in which one type can be followed through a considerable ex- 

 tent of geological time, the series of different forms by which the type 

 is represented at successive intervals of this time is exactly such as it 

 would be if they had been produced by the gradual modification of 

 the earliest form of the series. These are facts of the history of the 

 earth guaranteed by as good evidence as any facts in civil history. 



Hitherto I have kept carefully clear of all the hypotheses to which 

 men have at various times endeavored to fit the facts of paleontology, 

 or by which they have endeavored to connect as many of these facts 

 as they happened to be acquainted with. I do not think it would be 

 a profitable employment of our time to discuss conceptions which 

 doubtless have had their justification and even their use, but which are 

 now obviously incompatible with the well-ascertained truths of pale- 

 ontology. At present these truths leave room for only two hypoth- 

 eses. The first is that, in the course of the history of the earth, 

 innumerable species of animals and plants have come into existence, 

 independently of one another, innumerable times. This, of course, 

 implies either that spontaneous generation on the most astounding 

 scale, and of animals such as horses and elephants, has been going on, 

 as a natural process, through all the time recorded by the fossiliferous 

 rocks ; or it necessitates the belief in innumerable acts of creation re- 

 peated innumerable times. The other hypothesis is, that the succes- 

 sive species of animals and plants have arisen, the later by the gradual 

 modification of the earlier. This is the hypothesis of evolution ; and 

 the paleontological discoveries of the last decade are so completely in 

 accordance with the requirements of this hypothesis that, if it had not 

 existed, the paleontologist would have had to invent it. 



I have always had a certain horror of presuming to set a limit upon 

 the possibilities of things. Therefore, I will not venture to say that 



