One-tenth another, to nine-tenths me?”

“Soft is the breath of a maiden’s yes;
Not the light gossamer stirs with less;
But never a cable that holds so fast
Through all the battles of wave and blast,
And never an echo of speech or song
That lives in the babbling air so long!
There were tones in the voice that whispered then
You may hear to-day in a hundred men.”

When life steps into the world of matter there comes with it a sort of physical immortality, so to speak; not of the individual, it is true, but of the race. But the important thing to note is that the race is made up, not of a succession of wholly unrelated forms, but a continuation of the same kind of living organisms, and this sameness is due to the actual physical descent of each new individual from a predecessor. In other words, any living organism is the kind of organism it is in virtue of its hereditary relation to its ancestors.

It is part of the biologist’s task to seek a material basis, a continuity of actual substance, for this continuity of life and form between an organism and its offspring. Moreover, inasmuch as the offspring is never precisely similar to its progenitors he must determine also what qualities are susceptible of transmission and in what measure.

Ancestry a Network.—From the fact that each child has all of the ancestors of its mother as well as of its father, arises the great complications which are met with in determining the lineage of an individual. A person has two parents, four grandparents, eight great grandparents, and thus following out pedigree it is plain to be seen that through this process of doubling in each generation, in the course of a few centuries one’s ancestry is apparently enormous. By actual computation, according to Professor D. S. Jordan, if we count thirty generations back to the Norman invasion of England in 1066, at this ratio of duplication, the child of to-day would have had at that time an ancestry of 8,598,094,592 persons. But we know that the total number of inhabitants in England during the time of William the Conqueror was but a small fraction of this enormous aggregate. This means that we shall have to modify our inference that a child has twice as many ancestors as its parents; a condition which at first sight seems evident, but which is not literally true. The fact is that the parents of the child, in all probability, have many ancestors in common—a state of affairs which is brought about through the intermarriage of relatives, and this is especially frequent among remoter descendants of common progenitors. Time after time in genealogy strains of blood have crossed and recrossed until it is not improbable that a man of to-day who is of English origin has the blood in his veins from every inhabitant of England who lived during the time of William the Conqueror and left fruitful descendants. Instead of conceiving of ancestry as an ever branching and widening tree-like system as it recedes into the past, it is more accurate, therefore, to regard it in the light of an elaborate meshwork. The “family tree” in reality becomes the family net.

Ancestry in Royalty.—The pedigrees of royal families have proved to be of much importance in the study of human inheritance, not that royal traits are any more heritable than any other, but simply because the records have been carefully kept so that they are the most comprehensive and easily followed pedigrees available. The netlike weave of ancestry is particularly well exemplified in some of these families because of much close intermarriage. Their heritage typifies on an intensified scale the heritage of the mass of mankind. For example, if we go six generations back in the ancestry of Frederick the Great instead of the expected sixty-four individual ancestors we find only forty; or in a still more closely woven stock, in the Spanish royal line of Don Carlos we find in six generations instead of sixty-four individual ancestors, only twenty-eight. While the present German emperor might have had four thousand ninety-six ancestors in the twelfth generation back, it is estimated that owing to intermarriage he probably had only five hundred thirty-three.

Offspring Derived from One Parent Only.—So far in our reckoning of heredity we have counted elements from both father and mother, and the complications which arise from such a double ancestry are manifestly very perplexing ones. If we could do away with the elements of sex and find offspring that are derived from one parent only, it would seemingly simplify our problem very much for we should thus have a direct line of descent, free from intermingling. This, in fact, occurs to a greater or less extent among lower animals in a number of instances. There may be only female forms for a number of generations and the eggs which they produce develop directly into new individuals. Moreover, many of the simpler organisms have the power of dividing their bodies into two and thus giving rise to two new forms, each of which resembles the parent. This shows plainly that we may have inheritance without the appearance of any male ancestor at all, hence sex is not always a necessary factor in reproduction or heredity. The development of eggs asexually, that is, without uniting first with a male cognate, is termed parthenogenesis. The ordinary plant louse or aphid which is frequently found upon geraniums is a familiar example of an animal which reproduces largely in this way. During the summer only the females exist and they are so astonishingly fertile that one such aphid and her progeny, supposing none dies, will produce one hundred million in the course of five generations. In the last broods of the fall, males and females appear and fertile eggs are produced which lie dormant through the winter to start the cycle of the next year. Again, the eggs of some kinds of animals which normally have to unite with a male germ before they develop, can be made to develop by merely treating them with chemical solutions. The difference between an offspring derived in such a manner, and one which has developed from an egg fertilized by the male is that it is made up of characteristics from only one source, the maternal.

Dual Ancestry an Aid in Studying Heredity.—Although we have the factors of heredity in a more simplified form in the case of asexual transmission, as a matter of fact most of our insight into the problems of heredity has been attained from a study of sexually reproducing forms, because the very existence of two sets of more or less parallel features offers a kind of checking up system by which we can follow a given characteristic.

Reversion.—Occasionally, however, plants and animals do not develop the complete individuality we might expect, but stop short at or re-attain some ancestral stage along the line of descent, and thus come to resemble some progenitor perhaps many generations back of their own time. Thus it is well known that as regards one or more characteristics a child may resemble a grandparent or often some remote ancestor much more closely than it does its immediate parent. The reappearance of such ancestral traits the student of heredity designates as Reversion or Atavism.

Reversion may occur apparently in any class of plants or animals. It is especially pronounced among domesticated forms, which through man’s selection have been produced under more or less artificial conditions. For example, among fancy breeds of pigeons, there may be an occasional return to the old slaty blue color of the ancestral rock-pigeon, with two dark cross-bars on the wings, from which all modern breeds have been derived. This is almost sure to happen if the fancy varieties are inter-crossed for two or three generations. Another example of