106 SECOND ANNUAL REPORT OF 



from our species by being perfectly plain, with no black spots on the 

 wings. The body is black, and the front wings have a slight shade 

 of this color at their base, front edge, and tips. Its larva is pale green 

 [Fig. 76.] an( j feeds on various other cruciferous plants besides cab- 

 bages ; its chryalis (Fig. 7G) is also pale green or whitish, 

 regularly and finely dotted with black. 



This butterfly has existed from time immemorial on the 

 American continent, within the geographical limits already 

 given, and yet has never made its way into Missouri or any of 

 the southwestern States. Nor is it likely to ever do so ; and why? Be- 

 cause some insects are constitutionally incapacitated to live beyond 

 certain geographical limits. The range of an insect is governed by 

 various influences which I have not time to enumerate at present; 

 but the principal influence is undoubtedly climate — temperature — 

 heat. The " isothermal" lines, or the lines of equal heat, as all phys- 

 ical geographers are well aware, do not run parallel with the lines of 

 latitude, as one might at first thought suppose ; but if our isothermal 

 maps are to be relied on, vary most astonishingly to points north and 

 south of a given line. The same variation from a given line of lati- 

 tude is noticeable in the distribution of insects, or — to coin a word — 

 we have "isentomic," or iso-insect lines, which are as variable as the 

 lines of equal heat, by which they are doubtless to a great extent 

 governed. In Central Missouri we live on nearly the same latitude as 

 that of Southern Pennsylvania, and in North Missouri, as that of 

 Southern New York ; yet we do not live on the same insect line, but 

 nearly on that of Virginia and North Carolina, and even in the ex- 

 treme northern part of the State, a number of insects are found, 

 which on the Atlantic seaboard are never known to occur north of 

 Virginia,, and the same rule holds good with the birds and fishes of 

 the United States. The same thing is true of our Central and South- 

 ern counties. In other words many of our insects are southern, 

 not northern species, and as familiar examples, I might mention the 

 Tarantula of Texas (Mygale Hentzii, Girard), and its large Digger- 

 wasp enemy (Pepsis formosa, Say), which have been frequently 

 found in St. Louis county during the past two years, though they were 

 for a long time supposed to be confined to Texas. 



Now, since the indigenous Potherb Butterfly has never, in the 

 course of past ages, extended to any point South of Pennsylvania, 

 although its cruciferous food-plants have always flourished South of 

 that line, we are justified in concluding that it never will do so, and 

 that though a brood of the worms were introduced directly on to 

 some cabbage patch in the extreme Northern part of this State, they 

 would soon die out there. 



Consequently we have nothing to fear from this butterfly which 

 has always troubled our northeastern friends. But the case is very 

 different with another white cabbage butterAy which is now commit- 

 ting sad havoc to the cabbages in some parts of Canada, and some 



