218 The American Naturalist. [March, 
profoundly affect. Looking back into the past, it is the first 
climatic change which offers great differences of condition from 
the present. 
One of the consequences of the advance and retreat of the 
great ice sheet which once covered the northern latitudes of 
North America, is its influence on the distribution of terrestrial 
life. It is argued that the advance of this ice sheet must have 
forced southward hardy animals and plants which, when brought 
into competition with southern species in a struggle for existence, 
led, through inheritance, to important modifications in the general 
aspect or facies of the fauna or flora of any given region. Ina 
somewhat like way a retreat of the ice sheet towards the north 
may be supposed to have enlarged the area for life, and to have 
drawn with it those organisms which find colder latitudes more 
congenial to their lives, and thus opened a way to changes in the 
character of the life inhabiting the areas vacated by them. The 
survival of Alpine floras and faunas on mountain tops finds a 
ready explanation in a distribution brought about primarily by 
the latter of these causes, viz.: the retreat of the glacial ice sheet 
to the polar regions, and the resemblance of the conditions of 
those high altitudes to formerly existing in the valleys. 
In a discussion of the causes of the peculiar flora of the Santa 
Barbara Islands pointed out by Prof. Greene, Prof. LeConte? has 
adduced the aid of the glacial period and ascribed this pecularity 
to the survival of an old flora on the islands, while that of the 
neighboring continent has been more or less. modified by a strug- 
gle with hardy denizens forced into it by glacial conditions. His 
reasoning on this point seems to me cogent and conclusive in 
general, but not wholly adequate in the special case of the pecu- 
liarities of the flora of the Santa Barbara Islands. It is believed 
that he is right in the supposition that the present flora of Santa 
Cruz more closely resembles that which once existed on the 
contiguous coast than it does the present flora of the same locality. 
The remoteness of the continental glacier as indicated by its 
terminal moraine must have been great from the region under 
discussion, and the possibility of its influence on the equilibrium 
2 Amer. Journ. Arts and Sciences. Vol. XXXIV.; pp. 457-461. 
