52 LECTURES 



mammalia is, with equal truth, applicable to the entire 

 history of organic life upon the globe, from its very be- 

 ginning as far back as we have been enabled to trace it 

 down to the present instant. Our knowledge of the 

 life-system of the earliest Archaean time is exceedingly 

 meager, but when we come to examine the old primor- 

 dial oeaches of the Silurian epochs we meet with fossil 

 forms that are the representatives of the faume which 

 figured at or very near the dawn of life upon earth, They 

 are the primordial ancestors of all the main branches of 

 animals, except vertebrates, which are to be found in 

 recent times. They are the earliest, structurally the 

 simplest, remains known to the biologist. They are the 

 elements from which Nature has bulk up the vast num- 

 ber of complicated organisms of our own era, and it is 

 easily demonstrable that it was many millions of years 

 ago that they existed. And yet, even this more or less 

 varied fauna of Silurian time must have been derived 

 from still .simpler forms, the history of which reaches far, 

 far back into the vast unknown the unwritten pages of 

 geologic record. 



By an overwhelming number of facts, then, biologists 

 of our own and of past times have demonstrated beyond all 

 manner of doubt that the distribution of all animals in 

 space, or in other words the existing world's faunas, is in 

 accordance with certain known laws, and that distribu- 

 tion is wholly explained by the distribution of all ani- 

 mals that have existed in time; or in other words the ex- 

 liuct world's faunas which, in turn, were also distributed 

 over the various regions of the earth in accordance with 

 laws, also largely known to us, are essentially the same 

 as those laws which account for the modern distribution 

 of organic life. One of the most philosophic living think- 

 ers in biology tersely paragraphed our knowledge upon 

 this point when he said: 



"So long as each species of organism was 

 supposed to have had an independent origin, 

 the place it occupied on the earth's surface 

 or the epoch where it first appeared had 

 little significance. It was, indeed, perceived that 

 the organization and constitution of each animal or plant 

 must be adapted to the physical conditions in which it 

 was placed; but this consideration only accounted for a 

 few of the broader features of distribution, while the 

 great body of the facts, their countless anomalies and 

 curious details remained wholly inexplicable. But the 

 theory of evolution and gradual development of organic 



