932 ds Littorina litorea Introduced or Indigenous? [November, 
interesting are the known facts in regard to the distribution and 
spread in America of the shell we are considering, that an in- 
„quiry into the nature of these facts, and a search -for an explana- 
tion for them becomes a matter of more than special importance, 
The value of the settlement of the question as to whether Litto- 
rina iitorea has been introduced in recent times or is a native of 
America, is not limited to the settlement of this fact only. It has 
a broader value as well, inasmuch as it has a bearing upon the 
science of the distribution of animals. 
It must be remembered that no species of animal or plant can, 
in the strict sense of the word, be indigenous to both Europe and 
America. If such were the case it would be necessary to sup- 
pose that the two independent lines of descent, either from a 
common near or remote ancestor, culminating in the species, had 
a followed precisely identical courses of development. The latter 
- would require precisely identical conditions of environment—and 
such we know would not exist upon two separate continents. 
_ Hence a shell which is common to two continents must in some 
way have been introduced from one to another. It may be intro- 
duced by the agency of man, or by purely natural and physical 
causes, such as ocean currents, etc. For want of a better term 
noS he word zzdigenous has been used in the present paper to apply 
os to a species introduced in past time by natural ages and now 
ce thoroughly established as a resident. i 
e Sucha species is our so-called “ native periwinkle,” Littorina 
_ pailiata (Say). It is common to Europe, Greenland and America, 
_and has existed for a long time in all three countries, being found 
fossil in the Post-pliocene of all of them. It will be presently 
1own that this shell was probably introduced from the continent 
which it originated to the other by way of Greenland and Ice- 
_ land, and by strictly natural agencies. We therefore speak of 
; indigenous to America, though whether its descent from its 
parent species took place here or in Europe we are unable to say. 
But. we hope to be able to show that Littorina litorea did not exist | i 
America until introduced from Europe by man, and that SH 
| 1857. He considered it to be indigenous to 
for the reason thar “some of the oldest inhabit 
