according to their natural relations,
if you select one of them, and it results in making a leap (_saut
pardessus_) over to several others, you take another one of them a
little less remote; these two species, placed in comparison, will
then present the greatest differences from each other. It is thus
that we had begun to regard most of the productions of nature which
occur at our door. Then the generic and specific distinctions were
very easy to establish. But now that our collections are very much
richer, if you follow the series that I have cited above, from the
species that you first chose up to that which you took in the second
place, and which is very different from the first, you have passed
from shade to shade without having remarked any differences worth
noticing.

"I ask what experienced zooelogist or botanist is there who has not
thoroughly realized that which I have just explained to you?

"Or how can one study, or how can one be able to determine in a
thorough way the species, among the multitude of known polyps of all
orders of radiates, worms, and especially of insects, where the
simple genera of Papilio, Phalaena, Noctua, Tinea, Musca, Ichneumon,
Curculio, Capricorn, Scarabaeus, Cetonia, etc., etc., already contain
so many closely allied species which shade into each other, are
almost confounded one with another? What a host of molluscan shells
exist in every country and in all seas which elude our means of
distinction, and exhaust our resources in this respect! Ascend to
the fishes, to the reptiles, to the birds, even to the mammals, and
you will see, except the lacunae which are still to be filled,
everywhere shadings which take place between allied species, even
the genera, and where after the most industrious study we fail to
establish good distinctions. Does not botany, which considers the
other series, comprising the plants, offer us, in its different
parts, a state of things perfectly similar? In short, what
difficulties do not arise in the study and in the determination of
species in the genera Lichena, Fucus, Carex, Poa, Piper, Euphorbia,
Erica, Hieracium, Solanum, Geranium, Mimosa, etc., etc.?

"When these genera were established but a small number of species
were known, and then it was easy to distinguish them; but at present
almost all the gaps between them are filled, and our specific
differences are necessarily mi