The Principles of Biology, Volume 1 · Herbert Spencer
PROTOZOA
Infusoria Spongiadæ Gregarinidæ _Noctilucidæ_ Foraminifera _Thallassicollidæ_ In the woodcut, the dots represent orders, the names of which it is impracticable to insert. If it be supposed that when magnified, each of these dots resolves itself into a cluster of clusters, representing genera and species, an approximate idea will be formed of the relations among the successively-subordinate groups constituting the animal kingdom. Besides the subordination of groups and their general distribution, some other facts are indicated. By the distances of the great divisions from the general centre, are rudely symbolized their respective degrees of divergence from the form of simple, undifferentiated organic matter; which we may regard as their common source. Within each group, the remoteness from the local centre represents, in a rough way, the degree of departure from the general plan of the group. And the distribution of the sub-groups within each group, is in most cases such that those which come nearest to neighbouring groups, are those which show the nearest resemblances to them--in their analogies though not in their homologies. No such scheme, however, can give a correct conception. Even supposing the above diagram expressed the relations of animals to one another as truly as they can be expressed on a plane surface (which of course it does not), it would still be inadequate. Such relations cannot be represented in space of two dimensions, but only in space of three dimensions.
_Mammalia_ _Aves_ _Reptilia_ VERTEBRATA \ _Amphibia_\ _Pisces_ \ \ _Arachnida_ \ _Insecta_ \ _Crustacea_ \ \ Articulata \ | \ | _Myriapoda_ \ | \ ANNULOSA \ | \ |_Annelida_ \ _Scolecida_ \ | \ Annuloida \ | / \ _Echinodermata_ \ | / _Pteropoda_ _Cephalopoda_\ | | _Gasteropoda dioecia_ | | _Gasteropoda monoecia_ _Pulmonata_ | | \ | | MOLLUSCA------------- \ | | \ \ | | _Lamellibranchiata_ \ \ | | \ \ | | _Brachiopoda_ \ \ | | \ \ _Gregarinida_ Molluscoida------------ _Rhizopoda_ \ _Ascidioida_ _Polyzoa_ / \ / PROTOZOA / _Spongida_ _Infusoria_ _Hydrozoa_ / / COELENTERATA _Actinozoa_ § 100a. Two motives have prompted me to include in its original form the foregoing sketch: the one being that in conformity with the course previously pursued, of giving the successive forms of classifications, it seems desirable to give this form which was approved thirty-odd years ago; and the other being that the explanatory comments remain now as applicable as they were then. Replacement of the diagram by one expressing the relations of classes as they are now conceived, is by no means an easy task; for the conceptions formed of them are unsettled. Concerning the present attitude of zoologists, Prof. MacBride writes:-- "They all recognize a certain number of phyla. Each phylum includes a group of animals about whose relation to each other no one entertains a doubt. Each zoologist, however, has his own idea as to the relationship which the various phyla bear to each other.
"The phyla recognized at present are:-- (1) Protozoa. (2) Porifera (Sponges). (3) Coelenterata. (4) Echinodermata. { Cestodes. (5) Platyhelminthes { Trematodes. { Turbellaria. (6) Nemertea. (7) Nematoda. (8) Acanthocephala (Echinorhyncus). (9) Chætognatha (Sagitta). (10) Rotifera. (11) Annelida (Includes Leeches and Gephyrea, Chætifera). (12) Gephyrea, Achæta. { Tracheata (Peripatus, Myriapods, Insects). (13) Arthropods { Arachnids. { Crustacea. { Pycnogonida. (14) Mollusca. (15) Polyzoa (Including Phoronis). (16) Brachiopoda. (17) Chordata (Includes Balanoglossus and Tunicates. Some continental zoologists do not admit Balanoglossus)." [This last phylum of course includes the _Vertebrata_.] Though under present conditions, as above implied, it would be absurd to attempt a definite scheme of relationships, yet it has seemed to me that the adumbration of a scheme, presenting in a vague way such relationships as are generally agreed upon and leaving others indeterminate, may be ventured; and that a general impression hence resulting may be useful. On the adjacent page I have tried to make a tentative arrangement of this kind. At the bottom of the table I have placed together, under the name "Compound _Protozoa_," those kinds of aggregated _Protozoa_ which show no differentiations among the members of groups, and are thus distinguished from _Metazoa_; and I have further marked the distinction by their position, which implies that from them no evolution of higher types has taken place. Respecting the naming of the sub-kingdoms, phyla, classes, orders, &c., I have not maintained entire consistency. The relative values of groups cannot be typographically expressed in a small space with a limited variety of letters. The sizes of the letters mark the classificatory ranks, and by the thickness I have rudely indicated their zoological importance. In fixing the order of subordination of groups I have been aided by the table of contents prefixed to Mr. Adam Sedgwick's _Student's Text Book of Zoology_ and have also made use of Prof. Ray Lankester's classifications of several sub-kingdoms.
_Placental_-----+ | _Aves_ _Mammalia_----+ | _Arachnida_ | | | _Implacental_---+ | _Insecta_ | _Crustacea_ | | | | | VERTEBRATA | | | | | | | | | | _Reptilia_ _Chilopoda_ | | _Amphibia_ ARTHROPODA | | | | _Diplopoda_ _Pisces_ | | _Chætopoda_ _Cephalochorda_ | | CHORDATA ANNELIDA _Urochorda_ | | _Echiuroidea_ _(Tunicata)_ | | _Hirudinea_ +---------+ _Archiannelida_ | | BRACHIOPODA | | ROTIFERA _Dibranchiata_ | | | | _Cephalopoda_ | | +----+ _Tetrabranchiata_ | | | _Crinoidea_ MOLLUSCA ---------------+ | | | +------ ECHINODERMATA _Scaphopoda_ | | | | | _Asteroidea_ _Solenogastres_ | | | | | _Echinoidea_ _Gasteropoda_ | | | | | _Holothuroidea_ _Lammellibranchiata_----+ | | | | _Enteropneusta_ | | | | | | | | | | _Acanthocephala_ POLYZOA --------------------+ | | | | | +--- NEMATHELMINTHES | | | | | | | _Nematomorpha_ | | | | | | | _Nematoda_ | | | | | | | _Zoantharia_ Ctenophora _Rugosa_ _Acalephos_ _Alcyonaria_ COELENTERATA _Hydromedusae_ Actinozoa Hydrozoa | | | |-------- NEMERTEA | | | | +----------+ _Turbellaria_ ----------------| | PROTOZOA PLATYHELMINTHES _Corticata_ _Trematoda_ _Gymnomyxa_ _Cestoda_ | | | | Myxomycetes | _Triaronia_ | _Vobrocina_ _Calcarea_ _Foraminifera_ | PORIFERA Compound Protozoa _Demospongiae_ _Radiolaria_ Let me again emphasize the fact that the relationships of these diverging and re-diverging groups cannot be expressed on a flat surface. If we imagine a laurel-bush to be squashed flat by a horizontal plane descending upon it, we shall see that sundry of the upper branches and twigs which were previously close together will become remote, and that the relative positions of parts can remain partially true only with the minor branches. The reader must therefore expect to find some of the zoological divisions which in the order of nature are near one another, shown in the table as quite distant.
It is, indeed, a wholly gratuitous assumption that organisms admit of being placed in groups of equivalent values; and that these may be united into larger groups which are also of equivalent values; and so on. There is no _à priori_ reason for expecting this; and there is no _à posteriori_ evidence implying it, save that which begs the question--that which asserts one distinction to be generic and another to be ordinal, because it is assumed that such distinctions must be either generic or ordinal. The endeavour to thrust plants and animals into these definite partitions is of the same nature as the endeavour to thrust them into linear series. Not that it does violence to the facts in anything like the same degree; but still, it does violence to the facts. Doubtless the making of divisions and sub-divisions, is extremely useful; or rather, it is necessary. Doubtless, too, in reducing the facts to something like order they must be partially distorted. So long as the distorted form is not mistaken for the actual form, no harm results. But it is needful for us to remember that while our successively subordinate groups have a certain general correspondence with the realities, they tacitly ascribe to the realities a regularity which does not exist.
We saw (§ 56), that the broadest division among the functions is the division into "the _accumulation of energy_ (latent in food); the _expenditure of energy_ (latent in the tissues and certain matters absorbed by them); and the _transfer of energy_ (latent in the prepared nutriment or blood) from the parts which accumulate to the parts which expend." Now in the lowest animals, united under the general name _Protozoa_, there is either no separation of the parts performing these functions or very indistinct separation: in the _Rhizopoda_, all parts are alike accumulators of energy, expenders of energy and transferers of energy; and though in the higher members of the group, the _Infusoria_, there are some specializations corresponding to these functions, yet there are no distinct tissues appropriated to them. Similarly when we pass from simple types to compound types--from _Protozoa_ to _Metazoa_. The animals known as _Coelenterata_ are characterized in common by the possession of a part which accumulates energy more or less marked off from the part which does not accumulate energy, but only expends it; and the _Hydrozoa_ and _Actinozoa_, which are sub-divisions of the _Coelenterata_, are contrasted in this, that in the second these parts are much more differentiated from one another, as well as more complicated.
Besides a completer differentiation of the organs respectively devoted to the accumulation of energy and the expenditure of energy, animals next above the _Coelenterata_ possess rude appliances for the transfer of energy: the peri-visceral sac, or closed cavity between the intestine and the walls of the body, serves as a reservoir of absorbed nutriment, from which the surrounding tissues take up the materials they need. And then out of this sac originates a more efficient appliance for the transfer of energy: the more highly-organized animals, belonging to whichever sub-kingdom, all of them possess definitely-constructed channels for distributing the matters containing energy. In all of them, too, the function of expenditure is divided between a directive apparatus and an executive apparatus--a nervous system and a muscular system. But these higher sub-kingdoms are clearly separated from one another by differences in the relative positions of their component sets of organs. The habitual attitudes of annulose and molluscous creatures, is such that the neural centres are below the alimentary canal and the hæmal centres above. And while by these traits the annulose and molluscous types are separated from the vertebrate, they are separated from each other by this, that in the one the body is "composed of successive segments, usually provided with limbs," but in the other, the body is not segmented, "and no true articulated limbs are ever developed."
The sub-kingdoms being thus distinguished from one another, by the presence or absence of specialized parts devoted to fundamental functions, or else by differences in the distributions of such parts, we find, on descending to the classes, that these are distinguished from one another, either by modifications in the structures of fundamental parts, or by the presence or absence of subsidiary parts, or by both. Fishes and _Amphibia_ are unlike higher vertebrates in possessing branchiæ, either throughout life or early in life. And every higher vertebrate, besides having lungs, is characterized by having, during development, an amnion and an allantois. Mammals, again, are marked off from Birds and Reptiles by the presence of mammæ, as well as by the form of the occipital condyles. Among Mammals, the next division is based on the presence or absence of a placenta. And divisions of the _Placentalia_ are mainly determined by the characters of the organs of external action. Thus, without multiplying illustrations and without descending to genera and species, we see that, speaking generally, the successively smaller groups are distinguished from one another by traits of successively less importance, physiologically considered. The attributes possessed in common by the largest assemblages of organisms, are few in number but all-essential in kind. Each secondary assemblage, included in one of the primary assemblages, is characterized by further common attributes that influence the functions less profoundly. And so on with each lower grade.
If, as an alternative, this peculiar subordination of traits which organic forms display be ascribed to design, other difficulties suggest themselves. To suppose that a certain plan of organization was fixed on by a Creator for each vast and varied group, the members of which were to have many different modes of life, and that he bound himself to adhere rigidly to this plan, even in the most aberrant forms of the group where some other plan would have been more appropriate, is to ascribe a very strange motive. When we discover that the possession of seven cervical vertebræ is a general characteristic of mammals, whether the neck be immensely long as in the giraffe, or quite rudimentary as in the whale, shall we say that though, for the whale's neck, one vertebra would have been equally good, and though, for the giraffe's neck, a dozen would probably have been better than seven, yet seven was the number adhered to in both cases, because seven was fixed upon for the mammalian type? And then, when it turns out that this possession of seven cervical vertebræ is not an absolutely-universal characteristic of mammals (there is one which has eight), shall we conclude that while, in a host of cases, there was a needless adherence to a plan for the sake of consistency, there was yet, in some cases, an inconsistent abandonment of the plan? I think we may properly refuse to draw any such conclusion. What, then, is the meaning of these peculiar relations of organic forms? The answer to this question must be postponed. Having here contemplated the problem as presented in these wide inductions which naturalists have reached; and having seen what proposed solutions of it are inadmissible; we shall see, in the next division of this work, what is the only possible solution.
Text from Project Gutenberg, public domain in the United States.