18 ADAPTATION AND DISEASE 



the Algonkian limestones," 1 and that thus bacteria are to be 

 regarded as largely responsible for the oldest of all known sedi- 

 mentary rocks. 



THE GEOLOGICAL PARALLEL 



Bacteria have been there, almost ab initio, but this does not, 

 therefore, mean that all orders of pathogenic bacteria have 

 always been with us, or that while certain infections are of an 

 eminently respectable antiquity, this is necessarily true of all 

 zymotic disease. Recognizing the unity of natural phenomena 

 we must, I take it, hold that zymotic phenomena run parallel 

 with geological. I mean this, that certain species, and indeed 

 certain genera, have existed unchanged through countless ages 

 to the present day. To-day we find the brachiopod Lingula 

 living buried in the sand between the tide marks in the Tropics : 

 we find identical fossil remains in the Cambrian rocks, usually 

 (though wrongly) described as the earliest geological formations 

 in which fossil remains have surely been discovered. As I have 

 just pointed out, algae and bacteria are now known to antedate 

 them by a long period. Here, then, is an animal which has per- 

 sisted unchanged, over countless millions of years. The pearly 

 Nautilus, with its exquisitely chambered shell, is still found in 

 certain bays in the south-west Pacific : as Sir Archibald Geikie 

 declared : " This is a genus which has persisted through the 

 greater part of geological time," for identical chambered shells 

 are found as fossils reaching back to the Silurian epoch. Limulus, 

 the King crab, which is no crab but an arthropod, regarded 

 by Gaskell as in the line of ancestry of the vertebrates and 

 of man, is closely related to the Devonian Eurypterids, the 

 fossil remains of that period differing in but secondary characters 

 from the shells of Limulus of to-day. 



And so it is with shrimps like Anaspides 2 and yet other 

 crustaceans, and with fishes such as Ceratodus, the King fish 

 of Australia, which have remained apparently unaltered over 

 extraordinarily long periods of geological time. But while this 

 is so, we know equally well that during the same period other 



1 Walcott, C. D., "Precambrian Algonkian Algal Flora," Smithsonian 

 Miscellaneous Collections, vol. Ixiv. No. 2, 1914. 



1 I owe these latter examples to my colleague at McGill University, Professor 

 A. Willey, F.R.S. 



