INVASION FROM THE NORTH 163 



also, have only effected comparatively slight 

 modifications in the mimetic pattern already 

 produced under the influence of plexippus (see 

 pp. 168-72). It is impossible to feel equal 

 confidence in suggesting the line by which the 

 later invasion of the more tropical D. Berenice 

 took place ; but it is on the whole probable that 

 it too came by way of the north during some 

 temporary period of warmth. It is tolerably 

 certain that it did not invade North America 

 from the south. For although D. Berenice and 

 strigosa have produced as is shown above far 

 less change in the indigenous N. American 

 fauna than plexippus, they have still caused 

 distinct and perfectly effective modifications in a 

 single species ; whereas in South America their 

 representatives have not been shown to have had 

 any effect at all. It is probable that both the 

 American Danaidas as they pressed southward 

 were 'held up' for a considerable time at the 

 northern borders of the Neotropical Region, 

 unable at first to penetrate that crowded area. 

 Finally they burst their way through and are now 

 abundant throughout all the warmer parts of the 

 Region, the forms of plexippus extending further 

 into the temperate south, just as in the Northern 

 Hemisphere they range further north than those 

 of Berenice. We are made to realize the recent 

 date of the invasion of South America when we 

 remember that nowhere else in the world do 

 Danaine butterflies of equal abundance 'range 



