VI FLORA OF TASMANIA. 



companies! with a greater tendency to permanence in form : thus Acotyledons, Monocotyledons, and 

 Dicotyledons are an ascending series in complexity and in constancy of form. In Dicotyledons, 

 Salices, Urticecs, Chenopodiaeece, and other Orders with incomplete or absent floral envelopes, vary 

 on the whole more than Leguminosce, Lythracece, Myrtacece, or Rosacea, yet members of these pre- 

 sent, in all countries, groups of notoriously varying species, as Eucalyptus in Australia, Rosa in Europe, 

 and Lotus, Epilobium, and Rubus in both Europe and Australia. Again, even genera are divided : of 

 the last named, most or all of the species are variable ; of others, as Epacris, Acacia, and the majority 

 of such as contain upwards of six or eight species, a larger or smaller proportion only are variable. 

 But the prominent fact is, that this element of mutability pervades the whole Vegetable Kingdom ; 

 no class nor order nor genus of more than a few species claims absolute exemption, whilst the grand 

 total of unstable forms generally assumed to be species probably exceeds that of the stable. 



5. The above remarks are equally applicable to all the higher divisions of plants. Some 

 genera and orders are as natural, and as limitable by characters, as are some species; others 

 again, though they contain many very well-marked subordinate plans of construction, yet are so 

 connected by intermediate forms with otherwise very different genera or orders, that it is im- 

 possible to limit them naturally. And as some of the best marked and limited species consist of 

 a series of badly marked and illimitable varieties, so some of the most natural* and limitable orders 

 and genera may respectively consist of only undefinable groups of genera or of species. For instance, 

 both Graminece and Composites are, in the present state of our knowledge, absolutely limited Orders, 

 and extremely natural ones also ; but their genera are to a very eminent degree arbitrarily limited, 

 and their species extremely variable. Orchidea and Leguminosce are also well-liniited Orders (though 



small genera and the fact that monotypic genera seldom have their variations recorded in systematic works, but an 

 examination of his data and methods compels me to acquiesce in his statement. It has also been remarked (Bory de 

 Saint- Vincent, Voy. aux Quatre lies de P Afrique) that the species of islands are more variable than those of continents, 

 an opinion I can scarcely subscribe to, and opposed to Mr. Darwin's facts, inasmuch as insular Floras are characterized 

 by peculiar genera, and by having few species in proportion to genera. Bisexual trees and shrubs are generally 

 more variable than unisexual, which however is only a corollary from what is stated above regarding plants of simple 

 structure of flower. On the whole, I think herbs are more variable than shrubby plants, and annuals than perennials. 

 It would be curious to ascertain the relative variableness of social and scattered plants. The individuals of a social 

 plant, in each area it is social upon, are generally very constant, but individuals from different areas often differ much. 

 The Pinus st/heslris, Mughus, and uncinata are cases in point, if considered as varieties of one ; as are the Cedars of 

 Atlas, Algeria, and the Himalaya. 



* It should be borne in mind that the term natural, as applied to Orders or other groups, has often a double 

 significance ; every natural order is so in the sense of each of its members being more closely related to one or more 

 of its own group than to any of another ; but the term is often used to designate an easily limited natural order, that 

 is, one whose members are so very closely related to each other by conspicuous peculiarities that its differential cha- 

 racters can be expressed, and itself always recognized ; these may be called objective Orders ; OrcMdece and Graminece 

 are examples. Any naturalist, endowed with fair powers of observation and generalization, recognizes the close affi- 

 nity between a pseudobulbous epiphytical, and a terrestrial tuberous-rooted Orchid, or between the Bamboo and 

 Wheat, though the differences are exceedingly great in habit and in organs of vegetation and reproduction. Other 

 orders are as natural and may be as well limited, but having no conspicuous characters in common, and presenting 

 many subordinate distinct plans of structure, may be regarded as subjective. Such are Ranunculacece and Legu- 

 minosce, of which a botanist must have a special and extensive knowledge before he can readily recognize very many 

 of their members. No degree of natural sagacity will enable an uninstructed person to recognize the close affinity o' 

 Clematis and Ranunculus, or of Acacia and Oytisus, though these are really as closely related as the Orchids and 

 Grasses mentioned above. We do not know why some Orders are subjective and some objective ; but if the theory 

 of creation by variation is a true one, we ought through it to reach a solution. 



