THE LOGIC OF CLASSIFICATION. 245

the highest divisions was the heart. Linnaeus, accordingly, grouped thus : Heart, 2 ventricles, 2 auricles ; J" Living young, I. Mammalia. blood warm, red. Eggs, II. Aves (Birds). Heart, 1 vent., 1 aur. ; f With lungs, III. Amphibia. blood warm, red. Eggs, II. Aves (Birds), "eart, 1 vent., 1 aur. ; ( With lungs, III. Am blood cold, red. With gills, IV. Pisces (Fishes). Heart, 1 vent., aur. ; ( With antenna, V. Irisecta. blood cold, white. ( With tentacles, VI. Vermes (Worms). Now, as is well known, the heart is a very variable organ, and so is not well suited to give the great differentiating mark in the animal kingdom. It does not make the most of correlated properties, and it necessitates a great overlapping of classes. Later naturalists have, therefore, discarded it, and have given the place of honour to the nervous system. In this way they have been able to mark affinities and to dis- play gradations to a far greater extent than ever Linnaeus could, and to bring their classification nearer to what they conceive to be the ideal natural system, although there is yet much to be done before perfection is attained. By fixing on the nervous system as their chief classifying organ, they have fixed upon something that is of the highest scientific value. For what determines the value of an organ for classifying purposes ? The number of properties that it carries along with it. Presence of a nervous system, there- fore, means many things. It means, in the vertebrates, pos- session of a brain and spinal cord, shut out in a special cavity from the general visceral tube of the body, and situated opposite the side on which the limbs are placed. It means possession of an internal skeleton, as opposed to the exo- skeleton of such invertebrates as the lobster and the crab. It means possession of limbs jointed to the body, and always turned away from the nervous masses ; and these limbs never more than two pairs. It means possession of a heart (except in the case of the lancelet), as well as of a blood- vascular system, and blood (with one exception) of a red colour ; together with the peculiarity that the masticatory organs are " modifications of parts of the walls of the head, and are never modified limbs or hard structures developed in the mucous membrane of the digestive tube as they are in the invertebrates". It means, lastly, increase in intelligence, advance in mental endowment, the degree of advance depending on the size and weight of the brain, but still more on the brain's texture and convolutions. So that the Natural system has this great advantage over the Artificial that it is truer to the principles of natural science and of scientific classification in general ; it is more fortunate in