Jan. 3, 1884] 



NA TURE 



229 



definitions require it. If we knew all the known individuals which 

 have lived, we should have no species, we should have no genera. 

 That is all there is of it. It is simply a question of a universal 

 accretion of material and the collection of information. I do not 

 believe that the well defined groups will be found to run 

 together, as we call it, in any oae geological period, cer- 

 tainly in no one recent period. We recognise, however, that 

 they diverge to a wonderful extent ; cne group ha-; diverged at one 

 period, and another one has become diversified in a different 

 period ; and so each one has its history, some beginning farther 

 back than others, some reaching far back beyond the very begin- 

 ning of the time when fossils could be preserveil. I call attention 

 to this view because it is a very easy matter for us to use words 

 for the purpose of confusing the mind ; for, next to the power 

 of language to express clear ideas, is its power of expressing no 

 ideas at all. As we all know, v\e can say u.any things whicli we 

 cannot think. It is a very easy thing to .^-ay twice two is equal 

 to si.x, but it is impossible to think it. 



I would cite what I mean by variations of species in one of is 

 phases : I would just mention a genus of snakes, Ophibolus, 

 which is found in the United States. If we take the species of 

 this snake genus as found in the Northern States, we have a good 

 many species well defined. If we go to the Gulf Slates and 

 examine our material, we see we have certain other species wtU 

 defined, and they are very nicely defined and di-tinguished. If 

 now- we go to the Pacific coast, to Arizona and Xeu- Mexico, we 

 shall find another set of species well defined indeed. If we take 

 all these different types of our specimens of different localities 

 together, our species, as the Germans say, all tumble together ; 

 definitions disappear, and we have to recognise, out of the pre- 

 liminary list of thirteen or fourteen, only four or five. That is 

 simply a case of the kind of fact with which every biologist is 

 perfectly familiar. 



When we come to the history of the extinct forms of life, it 

 is perfectly trae then that we cannot observe the process of 

 descent in actual operation, because, forsooth, fos^ils are neces- 

 sarily dead. We cannot perceive any activities because fossils 

 have ceased to act. But if this doctrine be true we should get 

 the series, if there be such a thing ; and we do, as a matter of 

 fact, find longer or shorter series of structures, series of organisms 

 proceeding from one thing into another form, which are exactly as 

 they ought to be, if this process of development by descent had 

 taken place. 



I am careful to say this, because it is literally true, as we all 

 must admit, that the system must fall into some kind of order or 

 other. You could not collect bottles, you could not collect old 

 shoes, but you could make some kind of a serial order of them. 

 There are no doubt characters, by w hich such and such shoes 

 could be distinguished from other shoes, these bottles from 

 other bottles ; but it is also true that we have, in recent forms 

 of life in zoology and botany, irrefragable proofs of the meta- 

 morphose', and transformations, and clianges of the species, in 

 accordance with the doctrine which we commenced with. 



We now come to the second chapter of our subject. With 

 the assumption, as I take it, already satisfactorily proven, of 

 species having changed over into others — in considering this 

 matter of geological succession or biological succession, I bring 

 you face to face with the nature and mode of the change, and 

 hence we may get a glance, perhaps, at its laws. 



I have on the board a sketch or table which represents the 

 changes which took place in certain of the mammalia. I give 

 you a summary of the kind of thing which we fi d in one of the 

 branches of palaeontology. I have here two figures, one re- 

 presenting a restoration, and the other an actual picture, of two 

 extinct species that belong to the early Eocene period. One 

 represents the ancestor of the horse line, Hyracotherium, 

 which has four toes on his anterior feet, and three behind ; and 

 the other, a type of animal, Phenacodus, which antedated all the 

 horse series, the elepjhant series, the hog, the rhinoceros, and all 

 of the other series of hoofed animals. Each presents us with 

 the primitive position in which they first come to our knowledge 

 in the history of geological time. 



I have also arranged here a series of some leading forms of 

 the three principal epochs of the Mesozoic times, and six of the 

 leading ones of the Tertiary time. I have added some dates to 

 show you the time when the faunce which are entombed in those 

 lieds were discovered in the course of our studies ; and you will 

 easily see how unsafe it is to say that any given type of life has 

 never existed, and assert that such and such a form is unknow-n ; 

 and it is still more unsafe, I think, to assert that any given form 



of life properly defined, or that a specific intermediate form of 

 life, will not be found. I think it is much safer to assert that 

 such and such intermediate forms will be found. I have fre- 

 quently had the pleasure of realising anticipations of this kind. 

 I have asserted that certain types w ould be found, and they have 

 been found. You w ill see that I attend to the matter of time 

 closely, because there have been a great many things discovered 

 in the last ten or fifteen years in this department. In these 

 forms I give the date of the discovery of the fmna in w hich they 

 are embraced. 



Here we have the White River fauna discovered in 1856 ; then 

 we skip a considerable period of time, and the next one was in 

 1869, when the Cretaceous series was found. Six or seven Cre- 

 taceous fauna; have been found. Thus we have the Bridger 

 fauna in 1870, the Wasatch fauna in 1874. Next we have, in 

 1877, the Equus beds and the fauna which they embrace, which 

 also was found in 1878. The Permian fauna, which is one of 

 the last, is 1S79 ; and the last, the Puerco, which gives the 

 oldest and ancestral types of the modern forms of mammalia, 

 was only found in 18S1. When I first commenced the study of 

 this subject, about i860, there were perhaps 250 species knoun. 



