S U C C E S S I V E A P P R O X I M A T I O N I N DESCRIPTIVE ECOLOGY
61
apply to the dominants. Of eight species co-dominant on the floor of the
Pinetum oxalidosum four (Asarum europaeum, Asperula odorata,
Maianthemum bifolium and Trientalis europaea) probably developed in
the Tertiary under deciduous forest with dense summer shade, while
four (Lycopodium annotinum, Vaccinium myrtillus, Pyrolu secunda and
Linnaea borealis) are related to coniferous forests.
It is possible that ecological groups of species, rather similar t o
synusiae, may have had a common origin and have operated as units
throughout vegetation history but even this is doubtful and certainly
unproven.
Although historical evidence supports the existence of certain communities relatively unchanged through long periods of geological time,
it shows that this is by no means invariable. The history of many plant
communities is complicated arid major associations of the present day
may be drawn from elements in many different past communities.
Phylogeny proves therefore to be a very unsatisfactory and uncertain
basis for the classification of plant communities, although anyone who
studies the history and development of vegetation may draw very
valuable comparative data from a satisfactory classification of presentday plant communities.
Having rejected the phylogeny of plant communities as the basis for
classification, the problem is t o find means of comparing and arranging
phytocoenoses 011 the one hand and synusiae on the other into systems
that will throw into relief the relationship between their composition
and structure and the habitat.
c. D E F I N I T I O N O F C O M M U N I T I E S
In the discussion of classification given above it was suggested that
a natural classification should be based on all available characteristics,
and that a class should be defined by the “invariable presence of certain
common properties”. If we want t o distinguish vegetation units for later
correlation with habitat or with animal distribution, it is necessary that
the units should be distinguished by characteristics of the vegetation.
No features of the habitat should be used in the definition. If, however,
we want t o define vegetation-habitat complexes no such rest,riction is
necessary.
D. PHYSIOGNOMIC-ECOLOGICAL O R FLORISTIC CLASSIFICATION?
The purpose of a natural classification of vegetation is t o obtain a
definition and arrangement of plant communities which reflect as
closely as possible their relationship t o one another as determined by
their total response t o their environment. Communities which are
ecologically similar should be grouped together.
61
apply to the dominants. Of eight species co-dominant on the floor of the
Pinetum oxalidosum four (Asarum europaeum, Asperula odorata,
Maianthemum bifolium and Trientalis europaea) probably developed in
the Tertiary under deciduous forest with dense summer shade, while
four (Lycopodium annotinum, Vaccinium myrtillus, Pyrolu secunda and
Linnaea borealis) are related to coniferous forests.
It is possible that ecological groups of species, rather similar t o
synusiae, may have had a common origin and have operated as units
throughout vegetation history but even this is doubtful and certainly
unproven.
Although historical evidence supports the existence of certain communities relatively unchanged through long periods of geological time,
it shows that this is by no means invariable. The history of many plant
communities is complicated arid major associations of the present day
may be drawn from elements in many different past communities.
Phylogeny proves therefore to be a very unsatisfactory and uncertain
basis for the classification of plant communities, although anyone who
studies the history and development of vegetation may draw very
valuable comparative data from a satisfactory classification of presentday plant communities.
Having rejected the phylogeny of plant communities as the basis for
classification, the problem is t o find means of comparing and arranging
phytocoenoses 011 the one hand and synusiae on the other into systems
that will throw into relief the relationship between their composition
and structure and the habitat.
c. D E F I N I T I O N O F C O M M U N I T I E S
In the discussion of classification given above it was suggested that
a natural classification should be based on all available characteristics,
and that a class should be defined by the “invariable presence of certain
common properties”. If we want t o distinguish vegetation units for later
correlation with habitat or with animal distribution, it is necessary that
the units should be distinguished by characteristics of the vegetation.
No features of the habitat should be used in the definition. If, however,
we want t o define vegetation-habitat complexes no such rest,riction is
necessary.
D. PHYSIOGNOMIC-ECOLOGICAL O R FLORISTIC CLASSIFICATION?
The purpose of a natural classification of vegetation is t o obtain a
definition and arrangement of plant communities which reflect as
closely as possible their relationship t o one another as determined by
their total response t o their environment. Communities which are
ecologically similar should be grouped together.