Handbook of Paleontology 93 



suited to their environment. Sometimes a new form is 

 better fitted for the struggle than some local species ; it 

 thrives and continues to multiply at the expense of other 

 species until checked by limiting factors. Currents, then, 

 play their part in bringing new forms to other coasts and 

 in spreading those forms introduced along such coasts 

 so far as environmental conditions will permit. ^A case 

 has been noted of a new species of seaweed which sud- 

 denly appeared along the coasts of Cornwall and Brit- 

 tany, but within a few years had become completely 

 naturalized. An example of such a complete establish- 

 ment of a new species is seen along our own North 

 Atlantic shores in the common and very abundant Peri- 

 winkle (Littorina). Records show that this snail was 

 carried in ballast to the Canadian coast at Bathhurst, 

 N. B., in 1855 and from there it has spread down the 

 coast as far as New Haven. Its advance south of this 

 point has been checked by the warmer temperature of the 

 waters which has a fatal effect upon the floating egg 

 masses. The Acorn Barnacle establishes itself very 

 readily wherever it is carried and even colonizes on the 

 most wave- swept rocks and boulders, being most abun- 

 dant on the rocks between three-quarters and full tide. 

 It is therefore one of the first forms to appear on a rocky 

 shore severely battered by the waves, and the last to dis- 

 appear when conditions are severe, even surviving the 

 Rockweed. Studies along various sections of coast have 

 brought out the fact that certain forms seem to be more 

 or less constant and characteristic of those areas; but 

 there are other forms that vary quite considerably both 

 in distribution and in frequency. Certain tropic forms 

 of seaweeds have been found to have a periodic coloniza- 





