154 NATURAL SCIENCE . [September 



shown that it is possible to estimate age by tabulating the measure- 

 ments of a large number of individuals of the same species taken 

 together. The numbers fall into groups, or are concentrated about 

 nodes, each of which would therefore appear to represent the growth 

 of one year. Dr Petersen himself has applied this method to 

 fishes, but it may be used for some invertebrates, as recently shown 

 by Dr Th. Mortensen {Videnskabelige Meddelelser, 1897, pp. 319- 

 322). "Some invertebrates" we say, since Asterias rubens, Echinus 

 miliaria, Corbula gibba, Nucula nitida, and Carcinas maenas resisted 

 all Dr Mortensen's efforts to discover annual groupings. But he 

 was able to show that Solcn pelliicidus reached its full length of 

 23-28 mm. in two years, while those aged one year had a length of 

 13-17 mm. Similarly Ophioglypha tcxturata of one year old had 

 discs of 3-4 mm. diameter and arms 8-10 mm. long, while those 

 of two years old had discs of 7-11 mm. and arms of 20-32 mm., 

 and had reached sexual maturity but not yet done growing. 



Here we have a field of observation open to any sea-side 

 naturalist in want of work, and open also to collectors of fossils, 

 who might perhaps be able in this way to throw some light on the 

 number of years required for the formation of any given band of 

 fossiliferous rock. 



An Evolving Species 



Over and over again has it been objected to the theory of evolution 

 of species that no zoologist or botanist has been able to point to the 

 actual origin of a new species. Considering that the world has 

 existed many millions of years, and that men have studied species 

 for scarcely a century and a-half, the objection seems, on the face of 

 it, unreasonable. And yet it has been met more than once. Here 

 is one more instance of what may fairly be described as an evolving 

 species, an instance which has the additional merit of furnishing 

 time-data. 



On the north side of Dublin Bay is a tract of sand-hills known 

 as the North Bull, which owes its origin to the alterations caused 

 by the Dublin Harbour works, and certainly has not existed for 

 more than 108 years. This is inhabited now by a numerous 

 race of mice, agreeing in general form and in dimensions with Mus 

 musculus, but for the most part of a buff or yellowish-white tint, 

 and differing further from the norm of the species in that they 

 make burrows in the sand and construct nests at the bottom of 

 them. This race has been described in admirable detail by Mr H. 

 Lyster Jameson in the Journal of the Linnean Society {Zoology, vol. 

 xxvi., pp. 465-473, pi. xxx., 1898). He comes to the conclusion 

 that the colour, which closely resembles that of the sand-hills, is a 

 protective adaptation due to the fact that the short-eared owls and 

 hawks which frequent the North Bull pick out the darker mice, 



