from without, or sensation; sensibility as active, or excitatory of the functions of organs, reflex or motor action. The differentiation of the nervous system corresponds with this distinction. In the lower Mollusca the distinction between the two functions is little seen; but in Gastropoda, for instance, and still more in Cephalopoda, special organs are differentiated for sensation, and the nervous system is in communication with each of the differentiated sets of organs, stimulating and directing their activity. All this differentiation is associated with the distinction of polarity of motion; the nervous system is essentially co-ordinative, and binds together the activity of organs in the way of compensating for the separation of parts due to their differentiation and development in size. The nervous system compensates for separation of the functional activities of the organism, and the circulatory system compensates for separation of the physical parts of the body of the organism, maintaining unity for the organism co-ordinate with the physiological specialization and the morphological differentiation.

Branches, Classes, and Subclasses of Mollusca. Lankester's division of the Mollusca as a Phylum is first into two branches:

Branch A, the Glossophora, characterized by a "head region more or less prominently developed; always provided with a peculiar rasping tongue-the odontophore-rising from the floor of the buccal cavity;" and

Branch B, Lipocephala, of which the characters are "Mollusca with the head region undeveloped. No cephalic eyes are present; the buccal cavity is devoid of biting, rasping, or prehensile organs. The animal is sessile, or endowed with very feeble locomotive powers."

All these latter branch characters are practically negative characters: the Glossophora is a group formed of the Mollusca which possess in common a few important characters, and the Lipocephala are those which do not possess those characters. Only one class is recognized in the Lipocephala, i.e., the Lamellibranchia. The Glossophora comprise the three classes: first, Gastropoda, with two subclasses, (1) the Isopleura and (2) the Anisopleura; second, the class Scaphopoda; third, the

class Cephalopoda, with two subclasses, (1) the Pteropoda and

(2) the Siphonopoda.

The classes are chiefly distinguished by modifications of the foot, as is beautifully shown in Fig. 57.

FIG. 57.-Diagrams of a series of Mollusks to show the form of the foot and its regions, and the relation of the visceral hump to the antero-posterior and dorso-ventral axes. (1) A Chiton. (2) A Lamellibranch. (3) An Anisopleurous Gastropod. (4) Thecosomatous Pteropod. (5) A Gymnosomatous Pteropod. (6) A Siphonopod (Cuttle). A, P, antero-posterior horizontal axis; D, V, dorso-ventral vertical axis at right angles to A, P; o, mouth; a, anus; ms, edge of the mantle-skirt or flap; sp, sub-pallial chamber or space; ff, fore-foot: mf, mid-foot; hf, hind-foot; e, cephalic eyes; cd, centro-dorsal point (in 6 only). (After Lankester.)

In the Gastropoda the foot is simple, median in position, and flattened so as to form a broad, sole-like surface (No. 3). In the Scaphopoda the foot is adapted to burrowing life in the sand.

In the Pteropod the mid-foot is developed laterally into paddle-like swimming organs, and the fore-foot may be specialized into tentacles (Nos. 4 and 5).

In the Cephalopoda the fore, middle, and hind foot parts are separately specialized, the fore-foot merging with the head part and developing into arm-like processes, in some cases

beset with hooks or suckers, and the mid-foot is developed into a tube either closed or with lapping edges (No. 6).

Distinctive Features of the Lankester Classification.—The distinctive feature of Lankester's classification is seen in his descriptions of the subclasses. To show the nature of the characters selected as definitive of the divisions recognized, the chief characters of the subclass (2), Gastropoda Anisopleura, will be quoted, and for any further details the reader is referred to the fully elaborated and illustrated article in the Encyclopædia Britannica' on Mollusca.

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The definition includes the following characters, viz.:

"Gastropoda, in which, whilst the head and foot retain the bilateral symmetry of the archi-Mollusca, the visceral dome, including the mantle-flap dependent from it, and the region on which are placed the ctenidia, anus, generative and nephridial apertures, have been subjected to a rotation tending to bring the anus from its posterior median position, by a movement along the right side, forwards to a position above the right side of the animal's neck, or even to the middle line above the neck. . . . The shell is not a plate enclosed in a shell-sac, but the primitive shell-sac appears and disappears in the course of embryonic development, and a relatively large nautiloid shell (with rare exceptions) develops over the whole surface of the visceral hump and mantle skirt...

"The shell and visceral hump in the Anisopleura incline normally to the right side of the animal. . . . Atrophy of the representatives on one side of the body of paired organs is very usual." (p. 644.)

In these descriptions it will be noticed that characters chosen as distinctive are based upon comparison of the type under description with forms from which it is supposed to have been developed embryologically, or from which it is supposed to have descended by evolution.

The Gastropoda Anisopleura is conceived of as a Gastropod mollusk which has become modified in a particular way in the course of evolution.

CHAPTER XIV.

THE ACQUIREMENT OF

CHARACTERS OF GENERIC, FAMILY, OR HIGHER RANK ILLUSTRATED BY A STUDY OF THE BRACHIOPODS.

IN the foregoing chapters the history of organisms has been considered in its general principles.

We have noted how organisms are, in general, different for different periods of geologic time; how the peculiarities. of structure and function, which have led to their classification into many different classes, orders, families, genera, and species, are intimately associated with differing conditions of environment.

The steps by which the individual organism acquires its morphological and physiological characteristics have been examined, and the course of this development for each individual has been found to be determined by the ancestry from which it sprang.

The principles of classification have been discussed, and from the investigation in this direction it has been learned that each organic individual develops in the course of its individual growth not only the specific, but the generic, family, ordinal, class, and branch-characters of its parents. These characters have various rank in the classification; those which are of higher taxonomic rank are found to be of more ancient, and those of lower rank of more recent, geological origin. Therefore we may conclude, as a general law, that the lower the taxonomic rank of the character the shorter has been its life-period, i.e., the period of time through which it has been repeated by ordinary generation.

The various opinions regarding the nature of species have been discussed. All naturalists find the employment of species necessary to their science, though the exact definition.

of the term and the exact determination of any concrete species are difficult to accomplish.

The examination has revealed the fact that the fundamental difference in opinion regarding species turns upon the belief as to the mutability or immutability of species.

The idea that species are mutable is intimately associated. with the inquiry, What is the "origin of species"? In attempting to answer this question the deeper ones arise, i.e., What is evolved in evolution? and What is mutable?

The answer was that in any individual case all that is evolved is to be found in the variation exhibited in those characters by which it departs from the exact imitation of the characters of its ancestors, and that evolution consists in the acquirement of characters not possessed by the ancestors.

We examined the classifications of the Animal Kingdom particularly, and we found that, looked at analytically as composed of a vast number of different structures, or synthetically as a multitude of related organisms variously differentiated, and differentiated to various degrees along a few general lines of evolution, the Animal Kingdom is divisible into a number of definite groups, marked by definite organization, all the grander features of which were outlined in the Cambrian age, and the large majority of all the differentiations of even ordinal rank had been accomplished in the first quarter of the recorded history of organisms..

It is evident, therefore, that we must read the law of evolutional history in terms of the genera and species as they are distributed in families or in orders.

Generic and Specific Evolution Illustrated by the Brachiopoda. -In order to study the successive appearance of species and genera, it will be necessary to turn from the more general characters to the minuter marks distinguishing species from species, or at least genera from genera. For this purpose no better group of organisms can be selected than the Brachiopoda. In presenting the results of this analysis the paleontologist will miss that elaboration of the facts which would make the discussion of most practical use to him. The brief limits of this introductory treatise do not admit of this; and if the presentation of the facts shall stimulate some such readers.