1864. | British and American Conchology. 571 
tinctions, but at all events, it must be evident that discrimination 
between species is a matter of empiricism, and can only be accom- 
plished by a naturalist who has a thorough practical acquaintance 
with all the constituents of a genus in their various relations towards 
each other, and under all the circumstances of the life-history of each 
species. 
Dr. Lea has devoted a large share of his lifo to the attainment of a 
thorough acquaintance with the single conchological family Unionide. 
The Unios are not general favourites with shell collectors, perhaps 
because there is, to say the least, a strong family resemblance between 
all the species, and a good series requires the whole of a very capacious 
cabinet for its reception. Nevertheless, these fresh-water mussels 
have had afew enthusiastic admirers and collectors, from and before 
the time of Featherstone, whose book of travels in North America 
gives an amusing account of perils and hardships undergone in pur- 
suit of Unios. The tenth volume of Dr. Lea’s work on the Unionide 
contains a very valuable description of the soft parts and embryonic 
forms of many species, the shells alone of which had been previously 
described. It is a pity that the work is so strictly confined to techni- 
calities. Books of natural science will never gain their due respect 
from mankind till they openly recognize the fact that an accurate 
description of the habits and dispositions—in short, the biography of 
a living thing—is just as purely and as truly scientific as the most 
elaborate treatise on its physiology. 
The North American Unionide include, according to Dr. Lea, more 
than seven hundred species, whilst the rivers in Europe do not produce 
more than a dozen. 
The second volume of British Conchology, by Mr. Jeftrey’s, exhibits 
equally with the first, on the part of the author, a profuse expenditure 
of time and energy ungrudgingly bestowed on his favourite pursuit. 
In reading the book, it is easy to fancy oneself inhaling the fresh 
odour of the sea-shore, or of the sea-bottom ; turning a stone for a 
chiton, or poring over the dripping contents of a dredge in search of 
rissoz ; on the whole, however, the proportion of matter unattainable 
from other sources seems to be somewhat less in this than it was in 
the former volume. Many readers will, no doubt, think that the 
author has acted judiciously in abstaining from drawing inferences 
from his vast store of facts, either in favour of, or in opposition to, 
the theory of natural selection ; yet it is daily becoming more difficult 
to awaken any interest in matters which were the subjects of warm 
discussion only a few years ago. Even the discovery of a new species 
is less cared for: we want to know more of the old ones; for if Mr. 
Darwin’s theory be the correct one, there is not an animal or a plant of 
any species—far more than this, there is not a single character belong- 
ing to an animal or a plant of any species, but it has its own wondrous 
ancestral history to yield as a reward for patient study. On the other 
hand, if we regard “natural selection” as a mere conjecture, every 
part of every living thing may be examined as a witness to the proba- 
bility or the improbability of the grounds on which the conjecture has 
been made. All must admit that Mr. Darwin has fairly challenged 
VOL. I. 2a 
