i8o W. S. Cooper 



and rivers. Most of the interruptions are short and obviously in- 

 effective as barriers to coastwise migration. It is conceivable that 

 a break in the environment of considerable magnitude might be 

 effective. The Santa Lucia mountain range constitutes such a 

 possibility, because it presents a precipitous face to the sea fifty 

 miles in length, with very few and insignificant accumulations of 

 sand. At the northern end five species reach their southern limits 

 — Poa douglasii, Ely m us arenarius, Polygonum paronychia, 

 Lathyrus littoralis, and Franseria chamissonis. A similar but 

 less imposing stretch of cliffs in southern Humboldt and north- 

 ern Mendocino counties has no effect at all. 



North of the Canadian boundary the situation is very differ- 

 ent. Here, owing to the abruptly mountainous coast and over- 

 steepening of slopes caused by glaciation, suitable habitats for 

 strand plants are the exception rather than the rule; they are of 

 limited extent and separated by wide distances. The abrupt drop 

 in species density near the glacial boundary is surely significant. 

 Species from the south have invaded or reinvaded the glaciated 

 area for a slight distance only or not at all. No more than two 

 species dependent on accumulating sand — Car ex and Glehnia — 

 have gone far north, and these, so far as collections show, have 

 established themselves at only a few points. Elymus, Honc\enya, 

 Fragaria, Lathyrus, and Mertensia, requiring merely beach con- 

 ditions, are much more thickly distributed. Fewness and wide 

 separation of favorable habitats, together with shortness of avail- 

 able time, has certainly been effective in hindering northward 

 migration. Climate can have borne no causal part in this, for 

 there is no sudden change in any factor corresponding with the 

 abrupt drop in species density. 



Climate, however, must be effective in a more general way in 

 limiting the ranges of species, here as elsewhere. Freezing tem- 



