158 OEIGIN OF LIFE IN AMEEICA 



desci-ibes 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 

 liebraska. Mr. Shimek * likewise expressed the opinion that 

 the still existing northern colonies of Helicina occulta ap- 

 pear to be the remnants of a onoe 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 found 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 Palls. 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. HandlirschjJ 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. 



X Handlirsch, A., " Die Fossilen Insekten," p. 1191. 



