RELATIONSHIPS OF METASPERMAE. 609 



ated the characters which had begun to emerge under previous 

 glacial epochs. It has been shown how the Metachlamydeas 

 have been developed under the movements of plant-populations 

 attendant upon glacial encroachments. It has been indicated 

 how such a family as the Compositas have derived their modern 

 supremacy in the Middle North American region from their 

 ability to move quickly among the flying or advancing plant- 

 migrants. It remains to indicate the effect of resistance, topo- 

 graphical, climatic and biological, to such movements. As a 

 group of plants began to move southward before the glacier 

 they would find themselves opposed by rivers, hills and plains. 

 Those at home on the hill would be interfered with by the plain, 

 and vice versa. Again, the climate would doubtless change from 

 latitude to latitude, although perhaps the general northern ad- 

 vance of the ice, by modifying the climate, would assist the 

 south-bound plants by presenting conditions progressively 

 more difficult for the south-established plants with which the 

 south-bound plants were forced into competition. Lastly, a 

 constantly new group of aboriginal plants, already established 

 in southern regions, would oppose the entry of the south- bound 

 forms to their territory. Thus any characters whatever which 

 might contribute to the strength of the species would have been 

 selected for perpetuation. Not only the pappus of the dandelion 

 flower- fruit and the hooks of the Bidens achene would be seized 

 upon for the protection of the spesies from extinction, but the 

 shortening of the floral axis, the grouping of leaves to best 

 catch the light, colors that might attract some insect allies, 

 height, the increase or decrease in the size of the seed, all 

 would, if advantageous, be imprinted on the species, and varia- 

 tions would ultimately arise sufficient to justify the grouping 

 of the modified plants in categories different from those of the 

 original plants. Under such stress it is easy to see how the 

 raceme of flowers became shortened into the disk-like head, 

 how the heads at first in different planes, or racemosely 

 arranged, came to be compacted into the coryraboid group of 

 inflorescences, such as that of Solklago rigida. In every way, 

 the general passage from indefinitenee s to definiteness, in 

 structure, form, physiology, habitat, distribution, would be a 

 result of the enforced migrations. As factors in the evolution of 

 plants we must admit that, for the northern hemisphere in par- 

 ticular and for North America most particularly, the ancient 

 and repeated glaciations were of the utmost importance. 



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