The Biology of Fishes

By

M.Harry Kyle

Preface 

IN studying the life and characters of fishes one must face the problem of interpreting the phenomena. They may be taken as evidence of this or that theory, or they may be described as hard, cold facts without reference to any theory. A middle course may be suggested. One may form tentative deductions based on the facts and contrast them as impartially as possible with the conventional theories, so that the "fittest" may survive.

The possibility that characters have arisen at variance with the innate tendencies of "Heredity," and persisted without the aid of "Natural Selection," is a thesis which requires some courage to state. Yet this is one of the deductions that come, it seems to us inevitably, from a study of fishes, and a few words of explanation may be given here.

Fishes occupy a peculiar position in the hierarchy of animal life. We cannot be sure whence they came, but many of the important, even the essential, characters of Higher Vertebrates make their first definite appearance among them, and it is certain that fishes were the forerunner of all higher forms of life. We have these important Vertebrate characters, then, at the upper end, and nothing but a hypothetical form-very different from a fish and possibly a mutation from some larval form of lower degree -at the beginning. How did such characters first arise ? Which means, how can we conceive their origin?

An answer may be approached from two different sides. In the development of forms at the present day each individual, to begin with, is little more than a muscular body with nerves. The earliest fossil remains of fishes,

Dating back some 400 million years, are of the same nature. The unfolding of the characters follows the same general

lines in the development of the primitive forms of the present day and in the phylogenetic succession in the past history. But, while we may say that the unfolding of the characters in development is due to the inherited tendencies, such an interpretation would not apply to their original appearance unless we very considerably widen our ideas of heredity. As an example to be given Presently will show, the reactions of an organism under the stress of new conditions are more elemental than specific. And it is just because they stand at the beginning of a new series that fishes present us with more examples of this elemental power than any other group of bodied animals

In the making of form and structure, we can recognize a trinity of forces: inheritance, activity, and environment. For the reason stated, it is probable that inheritance is of less importance among fish than in higher animals. The greater variability of their characters also indicates this. And experiments show that the characters are very sensitive to changes in the external conditions. The constancy of a species, in fact, depends upon the persistence of a normal environment. We might almost include the latter under the term inheritance, were this usage not confusing to the student.

The activity of the individual, the link between inheritance and environment, is probably the most important factor in the making and remaking of characters. Under normal circumstances, its importance is masked, but under altered conditions, there is nothing left but this activity to account for the appearance of new characters. The plastic material that contains the traditions of the past responds in an elemental manner to the new impressions, and the resulting characters may be, have frequently been, of such a totally different appearance that the genetic relation cannot be discerned

One also has to face the question, whether Natural Selection, the survival of the fittest and elimination of the Unfit has been the directive principle in differentiation and evolution. Does this theory give a reasonable explanation of the facts?

The author's attitude towards the theory may be illustrated by quotations from the writings of Charles Darwin.

"The theory of natural selection implies that a form will remain unaltered unless some alteration is to its benefit." Again, "when we descend to details, we cannot prove that a single species has changed, nor can we prove that the supposed changes are beneficial, which is the groundwork of the theory. Nor can we explain why some species have changed and others have not."

Most biologists of the present day would probably agree with these words of Darwin. The theory of Natural Selection cannot be demonstrated as an explanation of evolution in the past, and we find ourselves unable to use it in unraveling the sequence of detailed events that led to the formation of species in the present. Darwin also admitted the possibility that new groups of animals have been formed "through the nature of the organism and of surrounding conditions, but not through natural selection." These statements apply with special force to fishes, and it is not difficult to point to numerous cases where changes have been quite the opposite of beneficial, and yet the species have survived and multiplied

One of these may be noted here by way of illustration. The sunfish, when a tiny larva goes through a condition which may well be compared to dropy. The natural course of development comes to a stop; the ordinary muscles and the nerve cord degenerate, or at least do not develop, and the larva rolls about in the water for some time like a helpless drop of jelly. But the life is not extinguished; the form changes again, new muscles replace the old, new nerve connections are established, and the sunfish persists to become one of the largest of fishes with a potential fecundity greater than anything known elsewhere in the animal kingdom. Examination of the characters of the adult sunfish affords no clue to its ancestors

The deduction noted above is strengthened by this and many similar instances. If the activity of ill-balanced fishes has enabled them to escape from a fate to which they were apparently doomed both by heredity and by natural selection, may we not believe that the living organism possesses a power which is superior to either?

How this deduction enables the phenomena of life to be interpreted in the case of fishes will be illustrated in the following pages.

Contents

-THE GENERAL CHARACTERS OF FISHES

<!--THE HABITS OF FISHES IN GENERAL

<!--MIGRATION OF FISHES

<!--THE DEVELOPMENT OF FISHES

<!--REGULATION OF THE FORM AND STRUCTURE:

<!--ECONOMY OF THE BODY. I. PRODUCTION AND TRANSPORT OF ENERGY

-ECONOMY OF THE BODY.

-VARIATION AND DIFFERENTIATION OF FISHES.

-THE GENEALOGY OF FISHES

<!--DISTRIBUTION OF FISHES IN TIME AND SPACE

<!--ADAPTATIONS TO SUIT PARTICULAR CONDITIONS

<!--FISHES AND THE WEB OF LIFE

- THE FOOD QUESTION

-THE MENTAL LIFE OF FISHES

<!--APPENDIX: SYNOPSIS OF THE FAMILIES OF FISHES

<!--BIBLIOGRAPHY:

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Prep. /Mona Mahmoud