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 which would permit of the greatest perfecting (that of the Vertebrates), a plan very different from those which she has been obliged to form as a preliminary to reaching it, one understands that, among the multitude of animals, one must necessarily come across not a single system of organisation which has become progressively perfected, but diverse very distinct systems, each of which has come into existence at the moment when each primary organ first put in its appearance" (p. 171). For Lamarck this order of Nature was not merely ideal--Nature had actually formed the classes successively, proceeding from the simpler to the more complex; she had brought about this evolution by transforming the primitive species of animals, raising them to higher degrees of organisation, and modifying them in relation to the environment in which they found themselves.