Изображения страниц
PDF
EPUB

A NEW THEORY

OF THE

ORIGIN OF SPECIES.

CHAPTER I.

PRELIMINARY

Classification of Animals and Plants-What are Species-Evolution of same kind, the prevailing idea.

The animal kingdom was classified by the late Prof. Louis Agassiz into five great branches or types, to wit: PROTOZOA, RADIATES, MOLLuscs, ARTICULATES and VERTEBRATES, according to a plan of structure peculiar to, and running through each.

Protozoa, is the name now given to a numerous family of nearly structureless forms of life, furnishing, as it were, the raw material from which the other great types have originated and diverged. The Moners constitute a large class of this type, of which Haeckel says-"They are not only the simplest organisms with which we are acquainted, but also the simplest living beings we can conceive of as capable of existing; and though their entire body is but a single, formless, small lump `of protoplasm, (each molecule of it being like the other), without any combination of parts, yet they

7

perform all the functions which in their entirety constitute in the most highly organized animals and plants what is comprehended in the idea of life: namely; sensation and motion, nutrition and propagation." (1–VIII–67).

Radiates include all animals whose organs radiate from a common center, and branch out into three classes-Polyps, Acalephs and Echinoderms.

Molluscs are all soft bodied animals, without articulated members, though sometimes containing hard parts internally, and are sometimes covered with hard shells. This type divides into three classes-Acephals, Gasteropods and Cephalopods.

Articulates are those having bodies more or less divided into lobes, rings or joints, with a skin or crust, sufficiently hard to form an external skeleton. Of these there are three classesWorms, Crustaceans and Insects.

Vertebrates include those having two elongated cavities, one above and the other below a bony axis, called the back bone. The upper cavity contains the spinal cord which enlarges at one extremity into the brain; and the lower, the organs of respiration, digestion and reproduction. This type divides into five great classes-Fishes, Batrachians, Reptiles, Birds and Mammals.

Haeckel, however, divides the animal kingdom into seven types or tribes, to wit: 1, The Protozoa. 2, The Zoophytes (or plant animals). 3,

ied animals). 5, The Echinoderms (or star animals. 6, The Arthropoda (or articulated animals). 7, The Vertebrata. (38-1-46).

The division of the classes in each type, runs into orders, families, genera and species. There are wide differences between the types; less wide between the classes; and these differences continue to diminish as we descend to families, genera and species.

The other forms of life are vegetable, commencing in microscopic Diatoms, which furnish food for the humble Moner, and diverging thence to the magnificent growths of the forest.

The vegetable kingdom divides itself into two great branches-Phænogams, or flowering plants and Cryptogams, or flowerless plants. The Phænogamous are of two classes-Exogens and Endogens the former comprising all plants composed of pith in the centre, bark outside, and wood between the two; and growing by annual additions of concentric rings of wood; and the latter comprising all plants whose stems are not composed of concentric layers, but whose woody substance is distributed through the stems in threads and bundles.

The Cryptogamous comprise all the lowest forms of vegetation; and instead of flowers have some thing analagous thereto, and produce spores instead of seeds. Of these there are three classes -Thallophytes, comprising sea-weeds, lichens and fungi-Anophytes, comprising mosses and

leaves-and Acrogens including ferns and lychopods.

A genus is a group of animals comprising one or more species. Thus of the genus Equus, the Horse, Ass, Zebra and Quagga are considered as species. But what are species? Strange to say, this is a question which does not appear to be well settled; no definition yet given seems to have been universally satisfactory to naturalists. Darwin, who ought to be good authority, is by no means satisfied. He says: "Certainly no clear line of demarcation has as yet been drawn between species and sub-species-that is, the forms which in the opinion of some naturalists come very near to, but do not quite arrive at, the rank of species; or, again between sub-species and well marked varieties, or between lesser varieties and individual differences. These differences blend into each other by an insensible series; and a series impresses the mind with the idea of an actual passage." Again: "From these remarks it will be seen, that I look upon the term species as one arbitrarily given, for the sake of convenience, to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms." (2-52-3). (New Ed. from 6th English, 41-2).

According to this a genus may be regarded as the original type, and species as the varieties

« ПредыдущаяПродолжить »