444 ZOOLOGY 



(a) Genus-exclusively Old World, except when spread 



by man : burdock. 



(b) Genus native in America as well as Old World, 



(1) Species native in America : thistle (if it is 



one of the American species, as will prob- 

 ably be the case). 



(2) Species introduced into our region from 



Europe : dandelion. 



(c) Genus and species native only in America : 



cactus ; snowberry ; sunflower. 



So far, we are concerned with the historic factors. 

 How did these plants come there ? When a plant or 

 animal is found native only in a given region, we say 

 that it is endemic; so the cactus and snowberry are 

 endemic in America. We may ask, however, whether 

 it originated in the country where we now find it, or 

 came from somewhere else. In the former case it is 

 endemic in the strict sense, in the latter it may be 

 called precinctive, if we wish to note the distinction. 

 For example, the brightly colored snails of the Hawaiian 

 Islands are certainly endemic ; they are wholly different 

 from those found elsewhere, and from their number and 

 variety have evidently undergone considerable evolu- 

 tion on the islands. The redwood is now confined to 

 California, but fossil redwoods are found in many 

 other regions. There is no special reason for thinking 

 that California, where the tree is making its last stand, 

 was its original home. The distinction thus made is a 

 real and interesting one, but very often we are unable 

 to come to a definite decision. Even so, it may be 

 worth while to try to estimate the probabilities. 



5. Having now discussed our plants from one point 

 of view, we may take up the environmental factors. 

 Perhaps there is a marsh or wet meadow not far away, 



