RELATIONSHIPS OF METASPERMAE. 609 
ated the characters which had begun to emerge under previous 
glacial epochs. It has been shown how the Metachlamydeze 
have been developed under the movements of plant- populations 
attendant upon glacial encroachments. It has been indicated 
how such a family as the Composite 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. Asa 
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 beinterfered 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 spezies 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 corymboid group of 
inflorescences, such as that of Svlidugo rigida. In every way, 
the general passage from indefinitene:s to definiteness, in 
structure, form, physiology. habitat, distribution, would be a 
result of the enforced migrations. As factorsin the evolution of 
plants we must admit that, for the nortt.ern hemisphere in par- 
ticular and for North America most particularly, the ancient 
and repeated glaciations were of the utmost importance. 
—39 
