This species is by far the most common butterfly in California. The caterpillar is very variable in its coloration, and is so like that of _P. Atalanta_, in company with which it is frequently found, that I have never succeeded in finding any distinguishing characters. It feeds throughout the year on _Urtica_ and on several malvaceous plants, and has the habit of all its congeners, of hiding itself in a rolled up leaf. _Pyrameis Cardui_ L. This most cosmopolitan of all diurnals, affects here, in its larva state, several malvaceous plants, and also the genus _Gnaphalium_, and its relations; but the plant for which it shows the greatest predilection is _Silybum Marianum_, a plant which formerly did not exist here, and has only spread since 1852. It now forms thickets in the neighborhood of San Francisco, as well as near most of our inland towns, but has never spread to a great distance from settlements. _P. Cardui_ frequents the same localities, and I found the same species in Australia in the same relation to the same immigrated plant, _Silybum Marianum_. I know very well that _P. Cardui_ existed here, as well as in Australia, before the immigration of this Mediterranean plant; but still, it is a remarkable fact that this cosmopolitan butterfly, notwithstanding its ability to adapt itself to plants of the most different families, still clings with such tenacity to a cosmopolitan plant, to whose universal distribution it is perhaps much indebted for the wide range which it itself attains. Next to the cosmopolitan character of this plant, _P. Cardui_ owes its great extent probably to its many generations and certain irregularities in the time of the appearance of the perfect insect; so that small colonies of the species are not so liable to be destroyed by inclemency of climate or exceptional atmospheric agencies; for a being that exists at the same time in the four stages of the egg, larva, chrysalis, and imago, has more chances of escaping cataclysms and deluge than others that are all at one time in the same stage of existence. The extent of the influence exerted by the number of generations, and the irregularity of period, can be very clearly recognized by the circumstance that species with one generation are always the most local; for instance, certain _Meliteæ_, _Argynnidæ_, _Theclæ_, most of _Sphingidæ_, etc.; that also the cosmopolitanism develops in proportion to the number of generations, and attains its maximum in certain _Vanessidæ_, _Danaidæ_, _Pyrameis_, etc. _Pyrameis Hunteri_ Fab.