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, spécial 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.

CHAPTER XII.

RECAPITULATION AND CONCLUSION.

THE obscurity and complexity of the phenomena of heredity afford no ground for the belief that the subject is outside the legitimate province of scientific inquiry. The existence, in a simple and unspecialized egg, of the potentiality of a highly organized and delicately adjusted animal, with special functions, instincts and powers of adaptation, with the capacity for establishing and perpetuating harmonious relations to the changing conditions of the world around it, is certainly one of the most profound problems of the material universe.

The fertilized egg is one of the greatest wonders within our knowledge, but this is no reason for refraining from studying it.

If we believe that living things have become what they now are by a process of gradual evolution, and that they owe their characteristics to the influences to which they have been exposed in the past, we must believe that the properties of the egg are capable of explanation, as far as these determining causes are open to study.

If we accept the generalizations of modern science, and hold that an unicellular ovum is homologous with and is descended from a remote ancestral unicellular organism, and that its properties have been gradually acquired by the natural selection of favorable variations, we must believe that the origin of its properties is as much within our reach as the origin of species.

The most prominent characteristic of heredity is that

it may be brought about not only by the various forms of asexual reproduction, but also by the sexual union of two reproductive elements, each of which is homologous with the other cells of the body.

In the lower animals and plants the cells which thus unite with each other, or conjugate, are similar in form, and probably in function also; but in all the higher organisms the male cell is very different from the ovum in form, size, and structure, as well as its mode of origin.

The present structure of each organism is the resultant of two factors, which we may call adherence to type and adaptation to new conditions, or if the use of terms without teleological implications is desired, we may speak of them as heredity and variation, or we may follow Haeckel and call them memory of past experiences, and perception of new relations. The precise terms to be used is a matter of little consequence. The essential thing is the recognition of the fact that each organism is the resultant of two factors, and that evolution is two-sided. An animal is what it is because it has the power to hold on to the experiences and adaptations which fitted its parents for their place in nature, and the parents acquired those peculiarities in virtue of their powers to gradually adjust their structure and habits to their environment.

This is the morphological side of evolution. Looking at it from its dynamical or functional side, we notice that each step in the process of advancement has been reached by divergent specialization and by physiological division of labor. Animals diverge from each other by acquiring the power to occupy different fields, to procure and use different kinds of food, to exist in different media, etc., and the organs and tissues and cells of a highly specialized animal or plant are adapted to perform