E. V. Wulff —100— Historical Plant Geography 



direction of the replacement of winged bisexual forms first by winged 

 virgin forms and later by wingless virgin forms. Of insects of the 

 bisexual generation first the females become wingless and then the 

 males. 



Present-day plant-lice represent different stages in the evolution of 

 cycles of plant-hce: some are only at the beginning of this evolution; 

 others at its end. Such cycles develop only in temperate climes with 

 their different seasons. In the tropics, with their even climate, the 

 bisexual generation is lacking, and there remain only winged and 

 wingless asexual forms. Whenever during the history of the earth 

 there arose new groups of plants or into a given region there pene- 

 trated new plants from some other region, plant-hce have migrated 

 from their old hosts to new ones, if the latter were suitable for them, 

 e.g., from willows and poplars to maples, from birches to maples, from 

 Myricaceae and Juglandaceae to Tilia, to various Papilionaceae, etc., 

 and here on the new hosts they became transformed first into new 

 species and then into new genera. 



Such a migration from certain plant hosts to others leads eventu- 

 ally to heteroecism. On the primary host there develop only some of 

 the generations, including the bisexual generations and those asexual 

 generations developing from fertilized eggs that have wintered over 

 (so-called "fundatrices"), and on the new, secondary host there 

 develop the sunamer asexual generations that terminate in the for- 

 mation of winged sexual forms (sexuparae), which fly back to the 

 primary host to deposit their eggs. 



Consequently, in plant-lice, just as in rust fungi, the dying out of 

 their primary hosts in any region leads to the formation of microcyclic 

 forms, represented only by wingless and winged asexual generations. 

 For instance, in the case of the disappearance of Ulmus and Pistacia, 

 that serve as primary hosts of plant-lice, the latter are preserved on 

 the roots of cereals or other plants in the form of asexual generations. 

 Hence, on the basis of the distribution of microcyclic forms we may 

 judge as to the former distribution of their primary hosts. For ex- 

 ample, the species of plant-louse Forda formicaria has as its secondary 

 host the roots of various cereals. A microcyclic form is now distrib- 

 uted throughout the entire holarctic (Europe, Siberia, North America); 

 presumably in Tertiary times Pistacia terebinthus also had approxi- 

 mately the same area of distribution. 



Another plant-louse, Trifidaphis phaseoli, has as its primary host 

 Pistacia niutica and as its secondary host the roots of various dioecious 

 herbs. As a microcyclic form it is distributed from Central Asia and 

 Egypt through the southern part of eastern Europe and through 

 western Europe to Great Britain, Greenland, North America, and even 

 South America (it is not found in Siberia). This gives grounds for 

 assuming that in Tertiary times Pistacia mutica, or a form very close 

 to it, had approximately the same area of distribution. Pistacia hce 

 (microcyclic forms) are found also in Formosa, Java, Egypt, and 

 South Africa. Moreover, the finding of the microcyclic form of Tri- 

 fidaphis phaseoli on the shores of Greenland, taking into account the 

 fact that this plant-louse spreads very slowly, indicates, without 

 doubt, that it has existed in Greenland uninterruptedly from the time 



