1890.) Certain Peculiarities in the Flora. 223 
Greene, is the change which has come to Southren California by 
the desiccation of the land. The islands, less acted upon than 
the main, from position and the neighborhood of the ocean, have, 
on that account, preserved a flora possibly like the Pliocene once 
found on the adjoining continent. As the environment changed 
on the main-land many of the genera died out, but they still per- 
sist where conditions are less modified because offset by local 
causes. 
It does not seem necessary to suppose that the islands are 
remnants of a new centre of distribution of life, or that it is 
wholly explanatory of the pecularity of the flora to ascribe the 
extinction of the same plants which now exist on Santa Cruz to 
a struggle with hardy varieties forced southward by glacial cold, 
The drying up of the climate is a potent factor which had a great 
influence and is sufficient, with other causes, to bring about great 
changes. As high mountains may be regarded as preserving an 
Alpine flora left in a congenial position by the retreating glacier, 
so the Santa Barbara Islands may, in a somewhat different way, 
present us a life preserved in sheltered points by the drying up of 
the neighboring sections of the continent which has less strongly 
affected the islands than the main-land. 
But neither of these causes alone is adequate to explain the 
peculiarities of flora or fauna in any circumscribed locality. 
There are many influences at work, and causes even which may 
have their origin far away from the regions which they effect. 
To analyze this nexus of influences is next to impossible. Ina 
broad way we may say that the present facies of the fauna and 
flora of any circumscribed locality is primarily the result of en- 
vironment, and where environment changes organic forms must 
change, while, when it remains constant, less modification is the 
result. A marked climatic change such as followed the drying 
up of a great area of such extent as has taken place in our south-. 
west, leaves its mark on the organisms near and remote from it. 
Two questions must be answered positively to make our 
reasoning logical. Has a desiccation taken place? and is it as 
great on the islands as on the main? Both of these questions 
are believed to be capable of definite answers. Desiccation has 
