FAUNA AND FLORA OF CALIFORNIA. 37I 



to attack afterwards, in their Puccinia stage, grass and 

 cereals; Rhytisma and D^dalea spores carried in the 

 feathers of birds will be carried from tree to tree and by 

 a slow but sure process will kill the giants of the forest. 

 This is one of the causes that but seldom you find a healthy 

 Madrona tree in the neighborhood of cultivated land and 

 that the Sycamores at Niles Station are dying away. 



Up to this point it is easy enough to classify the pheno- 

 mena and derive them from a common source, but fur- 

 ther on the effects become modified by the consequences 

 of other changes that have taken place in the relation of 

 the different forms of organic life. The agency still ex- 

 ists and marches on although we loose sight of the Ariadne 

 thread, which we followed into the labyrinth of co-operat- 

 ing and antagonistic causes. We soon loose sight of the 

 wave of the streamlet after its having joined the waters 

 of the river bed. 



The circumstances mentioned here form only part of a 

 general system of changes that have taken place in or- 

 ganic life since the settlement of this country. 



One of the most remarkable features of these changes 

 is the degree of ability in the different species that form 

 part of our flora and fauna to adapt themselves to altered 

 circumstances, and the methods by which they adapt 

 themselves. 



We will consider here a few cases that were easy to 

 follow up : 



I. Danais Plexippiis, a butterfly, common nearly 

 through this whole continent and lately emigrated into the 

 Sandwich Islands, New Zealand and Queensland. The 

 insect was rare in the vicinity of San Francisco up to the 

 year 1856, when suddenly a great number of this showy 

 butterfly appeared in the streets of San Francisco. The 

 caterpillar feeds exclusively on different Asclepiadace- 



