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, Hchinus 
miliaris, Corbula gibba, Nucula nitida, and Carcinas maenas resisted 
all Dr Mortensen’s efforts to discover annual groupings. But he 
was able to show that Solen pellucidus 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 texturata of one year old had 
discs of 3-4 mm. diameter and arms 8-10 mm. long, while those 
of two years old had dises 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 
OveER 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, pl. 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, 
