CHAPTER VII.

GEOGRAPHICAL DISTRIBUTION: SPECIAL CONSIDERATION: THE ADJUSTMENT OF ORGANISMS TO EN

VIRONMENT.

Résumé. In the case of the Madreporarian corals it was observed that as geological time progressed new genera actually were initiated, and the succession of genera and the rate of their increase was seen to be definitely associated with succession of time. Likeness of structure and likeness of time, dissimilarity of form and separation in time, slowness or rapidity of initiation of new genera, and a particular geological period of time for each family, order, or class, are interpreted to mean that there is a definite relationship existing between differentiation of structure and passage of time. This we assume to be a law of the order of events, and we infer the general hypothesis that the form and structure of organisms of one geological period are in some measure determined by the form and structure of the organisms of the period immediately preceding.

This hypothesis involves two particular propositions:

(1) That each organism is genetically related to some preexisting ancestor whose form and structure were not exactly like its own.

(2) That the process of organic reproduction is not a stereotype process of repeating in the offspring the exact characters of the ancestry, but that the production of differences between the parent and offspring is a normal factor in the reproductive process, either continuously or occasionally in operation.

There is, however, another fact to be noted: the innumerable differences in the conditions of environment are more or less distinctly expressed by differences in the kinds of organisms associated with them. All kinds of animals are not

found in every place or condition, but in each particular kind of environment particular kinds of animals are found, and their living is more or less dependent upon those conditions. Hence we infer another general hypothesis:

(3) That the conditions of environment do in some measure determine the particular form and structure of each organism.

The Gastropoda Illustrate the Law of the Relationship between Organisms and Environment. In order to show more particularly how the differences of form (expressed by different species, genera, and families in scientific classification) are related to differences in the conditions of environment, a class of the Mollusca, the Gastropoda, may be examined in detail. This group of organisms is convenient for the purpose because of the full statistics already accumulated regarding the geographical distribution of its species.

Meaning of the Classification of Organisms. Without defining the morphological characters indicated by the classification, it is important to remember that zoological classifications are fundamentally based upon morphological differences, that organisms of two distinct classes present greater morphological difference than those of a single class, that lesser divergence in form is expressed by division of the class into subclasses, and that the animals of the same order present greater resemblance to each other than to those of different orders. Families are again subdivisions of the orders, and each family includes two or more genera, and the species of each genus are alike in their general form, differing only in some of the more minute details. Hence when we describe the peculiarities of the distribution of genera, we are expressing the law of association between the generic form and the conditions of environment indicated by the geographical distribution. Thus, the common sea-whelk, Buccinum undatum (Fig. 33), represents the class Gastropoda as contrasted with the Dentalium (Fig. 37), belonging to the class Scaphopoda, Hylæa, a Pteropoda or Chiton (Fig. 36), a representative of the class Placophora. The Gastropoda, Scaphopoda, Pteropoda, and Placophora together constitute that division of Mollusca called Glossophora, being alike in the possession of a more or less distinct head-portion of the body, and of a

well-developed tongue (radula), which is generally armed with minute denticles set in rows (Fig. 34). The other types

op

FIG. 33-A Gastropod, the common whelk, Buccinum undatum, showing the spiral shell on the back of the animal, its large flattened foot, distinct head with two tentacles, at the base of which are the eyes. The siphon si and the operculum op are special parts not found in all Gastropods.

A

ি

C

D

FIG. 34.-Examples of the dentition of Gastropoda, single transverse rows of the denticles of the lingual ribbon (radula), greatly magnified, of (A) Natica, (B) Nassa, (C) Pleurotoma, (D) Scalaria.

of Glossophora are adjusted to various conditions of environment, but for our purpose it will be better to confine our attention at present to the single type of the class Gastropoda.

Distinguishing Characters of the Class Gastropoda. The common external characters of all Gastropods are these, viz. : Head and sense organs well developed, the former often bearing tentacles; a ventral muscular foot and undivided mantle, which frequently secretes a plate-shaped, or spirally twisted shell. The paleontologist knows Gastropods by their calcareous, more or less spirally twisted, univalve shells. These Gastropods, of which several tens of thousands of species are described, are specifically adjusted to all kinds of conditions of environment, and are distributed from the bottom of the ocean to the tops of the mountains.

Zones of Environment in which Gastropods are Distributed.If we arrange their environmental conditions in tabular order we have the following series, viz. :

Ist. Abyss of the ocean, or an abysmal zone, extending from 500 metres, or 250 fathoms, to the lowest known depths of the ocean.

FIG. 35.-Schematic Mollusk. (After Lankester.) a, tentacle; b, head; c, margin of mantle; d, margin of shell; e, edge of body;, edge of shell depression; g, shell; c, cerebral ganglion; ge, pedal ganglion; gpl, leural ganglion; 4, osphradium; 7, ctenidium; k, reproductive pore; 7, nephridial pore; m, anus; and p, foot; r, cœlom; s, pericardium; 7, testis; u, nephridium; v, ventricle of heart; z/, liver.

2d. Zone of Brachiopods, or of deep-sea corals (72-500 metres, 50-250 fathoms).

3d. Zone of Nullipores, or of Corallines (27-72 metres, 15-50 fathoms).

4th. Laminarian Zone (low tide to 27 metres, 1-15 fathoms).

5th. Littoral Zone (between low and high tides).

6th. Brackish water, sea-shores above tide, where fresh and brackish waters are mixed, and where the surface may be exposed to the air part of the time.

7th. Fresh water, as in rivers and lakes.

8th. Amphibious conditions, fresh water and land. 9th. Land, the surface of the land, or in the air.

These are zones of environment, which express a series of varying conditions of light, of oxygen, of air, of moisture, of degrees of temperature, of pressure of the medium, of depth, of height.

Reasons for Selecting the Gastropods.-The Gastropoda are selected because of the wide range of adaptation expressed in their distribution, and because the statistics are particularly full. The classification found in Zittel's Handbuch is adopted, so far as nomenclature and inclusion of genera are concerned; but Gastropoda will be spoken of as of the rank of a class, the more common usage of zoologists,* and the morphologically specialized forms, the Chitons (Placophora, Fig. 36) and the Dentalia (Scaphopoda, Fig. 37), will be omitted from the true Gastropoda, as is done by Zittel, following Ihring and Lacaze Duthiers: the Pteropoda will also be omitted.

Peculiarity of the Divisions of the Gastropods as to Range of Adaptation. Ranking Gastropoda as a class, with the restrictions above mentioned, it will include the following four orders, viz.: Prosobranchia, Opisthobranchia, Pulmonata, and Nucleobranchiata (or Heteropoda). The whole of the Heteropoda are specialized in structure and restricted in distribution to the surface and upper parts of the ocean water, and structurally they may be ranked with the monotocardian Prosobranchs. Six living genera with about 50 species are known, and a few fossil genera are referred to this order. The Pulmonata (Fig. 38, 38a) are air-breathers, and (with the exception of the Siphonarida) are restricted in distribution to land and fresh water. Six thousand (6000) living and 700

* Lankester's classification is (Encycl. Brit., art. "Mollusca," p. 633):

Phylum mollusca:

Branch A, Glossophora.

Class 1. Gastropoda.

Br. a. Isopleura.
Br. b. Anisopleura.
Class 2. Scaphopoda.

Class 3. Cephalopoda.

Branch B, Lipocephala (= Acephala, Cuvier).

Class 1. Lamellibranchia (syn. Conchifera).

Br. a. Pteropoda.

Br. b. Siphonopoda.