158 
ORIGIN OF LIFE IN AMERICA 
describes a species from the Oligocene Silex beds of Tampa 
in Florida apparently related to a Helicina still inhabiting 
the Bahama islands. Considering that the genus Helicina is 
almost confined to tropical and semi-tropical countries, we 
may assume that it spread northward at a time when very 
mild climatic conditions prevailed in the northern United 
States, and that a few more hardy species have survived in 
isolated colonies wherever they could obtain sufficient pro¬ 
tection against frost. This view is confirmed by the fact 
that Helicina occulta, now an extremely rare shell, is abun¬ 
dant in the Pleistocene loess beds of Indiana, Iowa and 
Nebraska. Mr. Shimek * likewise expressed the opinion that 
the still existing northern colonies of Helicina occulta ap¬ 
pear to be the remnants of a once common race which is 
evidently dying out. 
A family which resembles the Helicinidae, in so far as it is 
largely confined to tropical and sub-tropical regions, is that 
of the Phasmidae.f They comprise orthopterous insects of 
the shape of a small twig, and hence are known as “ walking 
sticks,” also “ prairie alligators ” or “ stick-bugs ” in America. 
All the species foimd in the United States are wingless. They 
are thus not liable to accidental transport except perhaps by 
water. All are vegetable feeders, and over a dozen kinds 
inhabit the southern States. Among these walking-stick 
insects there is one which has a remarkably northern range, 
viz., Diapheromera femorata. I met with it on Goat Island, 
above the Niagara Falls. It has also been observed near 
Toronto, and several other places in southern Canada. We 
possess no fossil evidence of the geological history of the 
genus Diapheromera; nevertheless, the fact that the family 
Phasmidae was already represented in Jurassic times, accord¬ 
ing to Dr. Handlirsch,J justifies the assumption that Dia¬ 
pheromera originated and began to spread northward in pre- 
Glacial times, and that it may be regarded as a southern relict 
in its present northern habitat. 
Two well-known instances of survivals of southern species 
* Shimek, B., “ Helicina occulta.” 
t Caudell, A. N., “ The Phasmidae of the United States,” p. 874. 
f Ilandlirsch, A., “ Die Fossilen Insekten,” p. 1191. 
