198 DARWINIANA. 



the basis of the actual arborescent vegetation. Espe- 

 cially " a very considerable number of forms nearly 

 identical with tertiary forms now exist in America, 

 where they have found, more easily than in our [Eu- 

 ropean] soil less vast and less extended southward 

 refuge from ulterior revolutions." The extinction of 

 species is attributed to two kinds of causes ; the one 

 material or physical, whether slow or rapid ; the other 

 inherent in the nature of organic beings, incessant, 

 but slow, in a manner latent, but somehow assigning 

 to the species, as to the individuals, a limited period 

 of existence, and, in some equally mysterious but 

 wholly natural way, connected with the development 

 of organic types : " By type meaning a collection of 

 vegetable forms constructed upon the same plan of 

 organization, of which they reproduce the essential 

 lineaments with certain secondary modifications, and 

 which appear to run back to a common point of de- 

 parture." 



In this community of types, no less than in the 

 community of certain existing species, Saporta recog- 

 nizes a prolonged material union between North Amer- 

 ica and Europe in former times. Most naturalists and 

 geologists reason in the same way some more cau- 

 tiously than others yet perhaps most of them seem 

 not to perceive how far such inferences imply the doc- 

 trine of the common origin of related species. 



For obvious reasons such doctrines are likely to 

 find more favor with botanists than with zoologists. 

 But with both the advance in this direction is seen to 

 have been rapid and great ; yet to us not unexpected. 

 "We note, also, an evident disposition, notwithstanding 



