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The cases where it can be proved to have occurred are not very numerous, it is true, but there are enough of them to present a serious difficulty. On our view that an external change which acts upon a certain part of the body may cause variation in that particular part, the chances against the parallel modification of allied organisms are very greatly diminished, so much so that the occasional occurrence of such modifications might be expected. If such cases were the rule they would be equally fatal to the theory of natural selection, whether our theory of heredity were accepted or not; but the cases are very far from frequent.
General and Special Homology: and the Significance of Serial Homology, Symmetry and Polymorphism. We have, so far, been occupied in studying the evidence for the law of heredity which is afforded by the slight and recently acquired differences between the sexes of the same species, between the young and the adult, between domesticated and wild races, between the hybrid offspring of allied species, between reciprocal hybrids, etc.
The bearing of this law upon the more profound problems of morphology has hardly been referred to, for the field which we have examined, although we have passed over it very rapidly, has furnished material for a treatise of considerable length. The discussion of the general problems of morphology would require another volume of even greater length, for I believe, and hope to show in another place, that the acceptance of my view will lead to considerable change in our manner of handling these problems; and will so shift ourview as to remodel some of the fundamental principles of the science.
I believe that it will throw light upon many obscure and perplexing questions, such as the significance of symmetry and general homology, the origin of polymorphism, the definition of an individual or person, etc.; but the end of a book is not the place to enter upon a a new field, and I am forced to reserve this subject for future discussion, although I will now indicate very briefly the nature of this explanation.
The basis of modern morphology is the doctrine that homology indicates genetic relationship.
Homology is fundamental similarity of plan, as distinguished from difference or similarity in physiological function. For example a man's arm and hand are fitted for grasping, and a bird's wing for flight, and their different uses render them unlike each other in a superficial view, although there is below and behind this obvious difference a more deep-seated resemblance. The feathers which cover the bird's wing have the same histological structure and the same origin as the hairs upon the human arm; the skin which covers the limb has the same character in both cases; the wing, like the arm, has a supporting skeleton, which consists of a shoulder and upper arm, a forearm, a wrist, and a hand; the muscles have the same structure and the same general arrangement, and the way in which they are supplied with nerves and blood-vessels is the same.
This fundamental identity of structure which is obscured, but not destroyed by the difference of use, is homology. In a superficial view the wing of a bird resembles the wing of a dragon-fly more closely than it resembles a man's arm, but careful examination shows that the insect's wing is not a limb at all, but a peculiar outgrowth from the body. The resemblance between a bird's wing and an insect's wing is not an homology, and it has no morphological significance: it does not indicate that there is any close relationship between a bird and an insect.
It is sometimes difficult to determine whether a resemblance is an homology or not, and in simple cases we decide by asking whether it can be due to similarity of use. We know that a bird's wing is more closely related to" a man's arm than it is to an insect's wing, because the resemblance between the two wings is no more than we should expect in organs adapted to the same purpose; but there is nothing in the use of the arm and wing to explain what they have in common.
In cases too complicated to be settled in this simple way we appeal to embryology, and ask whether the resemblance becomes more marked or less marked when ,we study it in its younger stages. The arm and the wing are more alike in the embryo than they are in the adults, and the features which they share in common make their appearance earlier than their distinctive characteristics.
An homology then is a resemblance which is not due to similarity of use, and which is more conspicuous in the embryo than in the adult.
This is the doctrine of homology considered from its structural side; historically considered, an homology is a resemblance due to community of descent, as distinguished from one due to recent modification. The modern morphologist believes that the resemblances between a bird's wing and a man's arm are due to inheritance from a remote ancestor in which the limb had all the characteristics. which are common to the wing and arm; that during the evolution of birds and mammals along two divergent lines from this ancestral form, the distinctive features which fit the wing for flight and the hand for grasping have been gradually acquired.
The doctrine that homology is an indication of ancestral relationship, and that the past history of organisms can be traced by studying their anatomy and embryology, is the basis of the modern science of morphology.
Now there are two kinds of homology, special homology and general homology. Homologies between corresponding parts of different animals are known "as special homologies, and those between different parts of the same animal as general homologies. As examples of general homology we may instance the serial homology of a cray-fish, the bilateral symmetry of mammals, and the radial symmetry of a star-fish.
So far as structure goes the homology between a man's arm and a man's leg is precisely like the homology between his arm and a bird's wing. It is a resemblance which is not due to similarity of use, but to fundamental resemblance, and it is more marked in the embryo than it is in the adult, and we seem, at first sight, to be justified in concluding that, if special homologies indicate genetic descent, general homologies must also; and that if general homologies cannot be explained in this way, the explanation of special homologies cannot be accepted.
Mivart has pointed out that it is impossible to explain general homologies by attributing them to inheritance from a common ancestor, and he therefore concludes that special homologies do not prove genetic evolution. On the other hand many authors have held that since special homologies indicate descent, general homologies must have the same meaning, and this belief has led to such speculations as the attempt to trace the vertebrates back to an annelid with a number of equivalent segments, to trace the echinoderms back to a community of worms, and to trace the polymorphic siphonophores back to unspecialized communities of hydroids.
I hope to show in another place that the acceptance of my view of the nature of heredity enables us to avoid both of these results, since it shows that special homologies may be due to heredity of one sort, and general homologies to heredity of another sort. Since corresponding cells in the homologous parts of the body of any individual are derived from closely related parts of the egg, they may be affected by similar gemmules and may thus give rise to what Darwin calls analogous variations. This form of inheritance I propose to call ontogenetic heredity, to distinguish it from ordinary inheritance from an ancestor. I shall point out, in another place, that while special homologies are due to ordinary or phylogenetic heredity, that is, to descent from a common ancestor, general homologies are, in many cases, due to ontogenetic heredity; that special homologies are old, and that they indicate genetic relationship, and thus enable us to trace the origin and history of animals, while general homologies are, in many cases, new, and recently acquired by secondary modification, and they do not indicate ancestry.