other times leads us to consider as a variety of a certain
species individuals a little bit different, which others regard as
forming a separate species."[340]

For Lamarck, as for Darwin later, the chief problem was not the
evolution and differentiation of types of structure, but the mode of
origin of species.

Lamarck is at great pains to show how arbitrary are our determinations
of species, and how artificial the classificatory groups which we
distinguish in Nature. Strictly speaking, there are in Nature only
individuals, "... this is certain, that among her products Nature has in
reality formed neither classes, nor orders, nor families, nor genera,
nor constant species, but only individuals which succeed one another and
resemble those that produced them. Now, these individuals belong to
infinitely diversified races, which shade into one another under all the
forms and in all the degrees of organisation, and each of which
maintains itself without change, so long as no cause of change acts upon
it" (p. 41).

But there is a natural order in the animal kingdom, a progression from
the simpler to the more complex organisations, a natural _Echelle des
etres_.

This order is shown by the relation to one another of the large
classificatory groups, for they can be arranged in series from the
simplest to the most complex, somewhat as follows:--

1. Infusoria.
2. Polyps.
3. Radiates.
4. Worms.
5. Insects.
6. Arachnids.
7. Crustacea.
8. Annelids.
9. Cirripedes.
10. Molluscs.
11. Fishes.
12. Reptiles.
13. Birds.
14. Mammals.

But the order of Nature is essentially continuous, and the limits of
even the best defined of these classes are in reality artificial--"if
the order of Nature were perfectly known in a kingdom, the classes which
we should be forced to establish in it would always constitute entirely
artificial sections" (p. 45).

In the same way the lesser classificatory groups represent smaller
sections of the one unique order of Nature. Note that Lamarck's
_Echelle_ is in no way a morphological one, and was not intended to be
such. It is a scale of increasing physiological differentiation, and the
stages of it are marked by the acquirement of this or that new organ
(_cf._ Oken). "Observation of their state convinces one that in order to
produce them successively Nature has proceeded gradually from the
simpler to the more complex. Now Nature, having had in mind the
realisation of a plan of organisation