160 Outlook to Nature 



is an error to talk about species, or that there 

 are always intermediate forms. My argument 

 is that species are not original entities or starting- 

 points but that the groups which, for conven- 

 ience, we name as species, are the present-time 

 results of a long process of modification. 

 Eventually, we may limit and refine our defini- 

 tion of species and we may find a real physio- 

 logical basis ; but of course I now have in mind 

 the practice of the present day. 



The missing links. 



Those types which are most pronounced and 

 distinct are usually those in which evolution or 

 change is most nearly completed; and those 

 old types that are unique are the ones in which 

 has probably begun the slow decline that ends 

 in extinction. The mastodon has perished, 

 and his tribe is disappearing ; the giant conifers 

 of other ages are represented in the isolated 

 groves of sequoias (there were giants in those 

 days !) ; the tulip tree or whitewood, the sassa- 

 fras, the ginkgo, the scouring-rushes are now a 

 broken and ragged army slowly but surely 



