668 THE AMERICAN NATURALIST. [Vow XXXII. 
Asia, including those of Kamtchatka, Amurland, and Sagha- 
lin, and that of Japan, “ show no sign of a similar warmth, but 
rather point to a climate colder than that of the present day” 
(Kayser, p. 354).1 
The Tertiary was apparently also a time of more or less 
intercontinental migrations or interchange of life-forms, which 
crossed the oceans over so-called continental bridges. Bering 
Strait was at one time such a bridge, and to explain the geo- 
graphical distribution of certain forms, there is thought to have 
been a more or less continuous land connection between India 
and Africa, and between Africa and South America, and pos- 
sibly in the Eocene, between Australia and southeastern Asia. 
However hypothetical these continental bridges may be, we 
do know that Central America and the Isthmus of Panama 
were elevated at the end of the Miocene, and that the bridge 
thus formed between North and South America became an 
avenue for the interchange of mammals and other animals 
which materially modified the distribution of life in the south- 
ern and northern parts of our continent. 
The elevation of the West Indies took place at this date, 
and these islands were peopled from the South American 
coast. What we already know of the rapid evolution of mol- 
lusks, insects, and mammals on thesé islands shows how closely 
dependent variation and adaptation are on isolation as well as 
changed topographic and climatic features. 
These problems have been studied with great care in the 
Hawaiian Islands by Gulick, and more recently by Hyatt. As 
well stated by Woodworth: “With the development of the 
umbrella-shaped topography of the Island of Oahu the land 
shells have varied from a common ancestral coastal type to 
valley-cradled, differentiated varieties, in the upper and dis- 
jointed valleys of this dismantled, volcanic island cone.” ? 
The limits of this address do not permit us to treat at length 
1 It has also been claimed by J. W. Gregory that the fossil plants of the Green- 
land Miocene beds may have been drifted from the southward, and that the tem- 
perature of the polar region was not so elevated as Heer has led us to suppose 
(Nature, vol. lvi, 1897, p. 352-) 
2 The Relation between Base-Leveling and Organic Evolution: referring to 
T. T. Gulick’s article in Proc. Boston Soc. Nat. Hist., vol. xxiv (1870), pp- 166, 167- 
