, families, genera and species. The lowest group but
one is the "genus," which contains one or more different kinds termed
"species," as e.g., the species "wood anemone" and the species "blue
titmouse." The lowest group of all--a species--may be said to consist
of individuals which differ from each other only by trifling
characters, such as characters due to difference of sex, while their
peculiar organization is faithfully reproduced by generation as a
whole, though small individual differences exist in all cases.

The vegetal, or vegetable, kingdom, consists of the great mass of
flowering plants, many of which, however, have such inconspicuous
flowers that they are mistakenly regarded as flowerless, as is often
the case with the grasses, the pines, and the yews. Another mass, or
sub-kingdom, of plants consists of the really flowerless plants, such
as the ferns, horsetails (Fig. 1), lycopods, and mosses. Sea and
fresh-water weeds (_algae_), and mushrooms, or "moulds," of all kinds
(_fungi_), amongst which are the now famous "bacteria," constitute a
third and lowest set of plants.

[Illustration: FIG. 1. HORSE-TAIL (_Equisetum drummondii_).]

The animal kingdom consists, first, of a sub-kingdom of animals which
possess a spinal column, or backbone, and which are known as
vertebrate animals. Such are all beasts, birds, reptiles, and fishes.
There are also a variety of remotely allied marine organisms known as
tunicates, sea-squirts, or ascidians (Fig. 2). There is, further, an
immense group of arthropods, consisting of all insects, crab-like
creatures, hundred-legs and their allies, with spiders, scorpions,
tics and mites. We have also the sub-kingdom of shell-fish or
molluscs, including cuttle-fishes, snails, whelks, limpets, the
oyster, and a multitude of allied forms. A multitudinous sub-kingdom of
worms also exists, as well as another of star-fishes and their
congeners. There is yet another of zoophytes, or polyps, and another
of sponges, and, finally, we have a sub-kingdom of minute creatures,
or animalculae, of very varied forms, which may make up the sub-kingdom
of _Protozoa_, consisting of animals which are mostly unicellular.

[Illustration: FIG. 2. A TUNICATE (_Ascidia_).]

Multitudinous and varied as are the creatures which compose this
immense organic world, they nevertheless exhibit a very remarkable
uniformity of composition in their essential structure. Every living
creature from a man to a mushroom,