the struggle for life, the weakest perish; the strongest perpetuate themselves, and that the different surrounding external conditions are the agencies by means of which these effects are produced.

Classification.-I shall here merely give an abstract of the principal classifications that have been, from time to time, proposed.

I.

1. Animals possessed of blood (i. e., red blood). . This division corresponds to the vertebrata of modern writers.

2. Animals that were exsanguious or had a colourless fluid instead of blood. This division comprised the invertebrata of modern writers-(Aristotle).

II.

1. Animals having warm, red blood, and a heart consisting of four cavities.

a. Mammalia

b. Aves.

2. Animals having cold, red blood, and a heart having two cavities.

a. Reptilia.

b. Pisces.

3. Animals having cold, white sanies, and a heart containing a single cavity.

a. Insecta.

b. Vermes (Linnæus).

III.

1. Animals whose hearts contain four cavities. a. Mammalia.

b. Aves.

2. Animals whose hearts contain three cavities. a. Reptilia.

b. Amphibia.

3. Animals whose hearts contain two cavities. a. Pisces.

b. Most Mollusca.

4. Animals whose hearts contain only a single cavity.

a. Articulata.

5. Animals in which the functions, both of stomach and heart, are performed by the same organ, e. g., Medusæ―(Hunter).

.

IV.

1. Vertebrata.

2. Mollusca.

3. Articulata.

4. Radiata vel Zoophyta-(Cuvier).

V.

1. Spini-cerebrata vel Vertebrata.
2. Cyclo-gangliata vel Mollusca.
3. Diplo-neura vel Articulata.
4. Cyclo-neura vel Radiata (Grant).

This last division of the Animal Kingdom into subkingdoms is the most modern and most generally adopted, and will be followed in the ensuing pages. Each sub-kingdom is further sub-divided into classes, orders, sub-orders (sometimes), genera, species, and varieties; e. g., the Bull dog is a variety of the species canis familiaris, belongs to the genus canis; sub-order Digitigrade Carnivora; order Carnivora; class Mammalia; sub-kingdom Vertebrata.

OUTLINES

OF

ZOOLOGY

AND

COMPARATIVE ANATOMY.

VERTEBRATA.

CHAPTER I.
SUB-KINGDOM.-VERTEBRATA.

GENERAL DESCRIPTION.-Animals that possess a "notochord" or "chorda dorsalis" at some period of their existence, which is replaced before maturity by the vertebral column (in the Amphioxus the noto-chord is persistent throughout life)-skeleton, always internal: limbs, never more than two pair, sometimes absent: alimentary canal has a distinct inlet and outlet: portal and lacteal systems, always present: respiration is effected by distinct breathing organs, often assisted by the skin: heart contains two or more cavities (absent in Amphioxus): blood is red and corpusculated : nervous system is much better developed than in the highest of the Invertebrata: reproduction is always sexual, the sexes being in different individuals :

Most Vertebrates are Oviparous.