Heredity.

PAUL WATERMAN, M.D., Hartford.

The principle of the common origin of life was accepted as established by the work of Charles Darwin about fifty years ago, although for fifty or sixty years before his time this theory had been slowly developing in the minds of a number of men. It was finally based by Darwin upon the proposition that through natural selection there is a survival of the fittest. He believed that there was a constant variation among individuals of the same species and that natural selection chooses the fittest types from among the continually varying members of each group. In order to explain the apparently continuous evolution in any given line, he was compelled, although somewhat reluctantly, to adopt the principle of the inheritance of acquired characters,— that is, that the effect of parental experience was transmitted to the offspring. In order to have some process whereby he could explain the hereditary transmission of traits, he evolved the theory of pangenesis, according to which every single part of the body gives off minute buds or pangenes which pass through the blood to the reproductive organs so that each egg-cell and sperm comes to contain a complete set of these buds representing all parts of the body. This theory has never been proven and has little or nothing to support it in the body of our biological experience. The inheritance of acquired characters was seriously questioned by Weissmann, who presented as a substitute for pangenesis the theory of the continuity of germ-plasm. In Weissmann's own words the idea is that a part of the germ-plasm "contained in the parent egg-cell is not used up in the construction of the body of the offspring, but is reserved unchanged for the formation of the germ cells of the following generation." The two following diagrams will perhaps explain the meaning of these two theories: (Fig. 1.) (Ref. No. 1.)

Thus we see that the germ cells of each individual are derived directly from the germ cells of the preceding generation. This segregation of the germ cells at the beginning of development is not wholly an imaginary idea. It has already been seen under the microscope in several instances. As a result of this theory you will note that the connection between the characters of two successive generations is not direct, but indirect. The traits of the son do not depend directly upon the father's, but indirectly in the sense that both depend upon the nature of the germ-plasm from which both are formed, having similar material to start with and similar conditions in which to develop; like therefore tends to beget like.

That like does tend to beget like has been generally accepted, and the term "heredity" has been applied to the process through which the tendency expresses itself, but if it were an absolute fact that like begets like instead of merely a general tendency, there would be no science of heredity at all. If all individuals of a race were identical in type, the only problem of heredity would be to account for this identity. It is also a matter of common experience that no two things in nature are ever exactly alike. Some variations exist between the most similar parents and offspring, and it is the occurrence of these variations that offers our first problem in heredity. There are two general classes of variations, the continuous and the discontinuous. In grouping the various languages of the world for instance, we would find,

at least during the early stage of our investigation, that there is no smooth or sensible transition from one language to another, and we should call this a discontinuous variation, whereas in regard to their stature men pass by a fairly regular gradation from dwarf to giant in the form of a continuous variation. If we tabulate the statures of a thousand men ranging from sixty to seventy-six inches, we shall find that the largest number of men are about sixty-eight inches tall, and that the number falls off with about equal rapidity in either direction, which will give us what is known as the normal variability curve. (Fig. 2.) (Ref. No. 1.)

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FIG. 2.

If we were to toss sixteen coins a thousand times and class the results according to the number of heads that fall, we should get a curve of almost exactly the same appearance. We thus see that the occurrence of a certain number of men of common height within a constant community may be a matter of pure chance.

It has been assumed that inborn variations are due to changes in the germ-plasm, whereas modifications or acquired characters are usually described as the abiding effects of external conditions on the body or organism. We cannot, of course, consider any organism apart from the conditions under which it lives,

but we may say that inborn characters are expressed only under certain conditions of environment, and acquired characters can appear only when the organism possesses certain inborn tendencies to respond to outside conditions. Thus in a sense every character is both inborn and acquired. Given constant conditions of life within a particular species, the organism would show only its inborn characters. If we could take a pair of identical twins, separate them at birth, and rear one under the best of conditions and the other under the worst, the differences resulting would be purely modification differences; whereas if we take two children of different parentage and rear them under exactly the same conditions, the differences would be altogether inborn. The occurrence of these inborn variations from the parental type has long been a matter of study. It seems probable that a great majority of them are due simply to new combinations of previously existent characters. Suppose we take four sacks of marbles of different colors, black, green, red, and yellow, to represent the respective germ-plasms of four individuals. By mixing the black with the yellow and the red with the green we get two new germ-plasms. If we now take random samples from each lot, say one-half dozen at a time, and mix the two, we will get what corresponds to a new generation. These groups will differ from each of their parents, and they will also differ among themselves. Something of this sort is going on in heredity. Bi-parental inheritance is not an intimate blend of two substances. In our example we did not introduce any new kinds of marbles; it was only that the combinations of the different colors were new. Similarly in heredity it is believed that most inborn variations are due to re-combinations of previously existing characters. It is true, however, that in order to have a new combination we must start with actual differences. Our experiment with marbles would have meant nothing if they had all been white. How then do these differences arise? There have been many occasions in nature observed in which something new evidently made its appearance. This sudden appearance of a novelty Darwin calls a single variation, and it is nowadays called a mutation. I will recite two historical examples of this

phenomenon. In 1791 on the banks of the Charles River in Massachusetts a male lamb was born that differed, for no assigned reason, from its parents by having a short body and short bandy legs, as the result of which it was unable to jump over fences. It differed markedly from the ordinary sheep. The variation apparently arose spontaneously, but after the ram was used for breeding it was noted that its offspring were either pure Ancon, like the parent, or else pure ordinary sheep. This type proved to be of no real value and has long since died out. The earliest recorded mutation occurred in 1590, when in a certain garden in Heidelberg there appeared among a number of plants of celandine of the ordinary type a peculiar new form with lancinated leaves and petals. This cut-leaved variety breeds pure from seed and is now widely grown, but all the specimens existent are descended from this one plant which arose unexpectedly as a sport. De Vries found a large number of mutations occur in working with the evening primrose and within a relatively short time ten distinct new types were discovered, all but one of which bred pure from the seed. It is probable, therefore, that such differences or mutations are of frequent occurrence in all life, and that many of our varieties of plants and animals have arisen in this way. Ordinarily only the most marked novelties have been noted or recorded, but it seems plausible that small changes occur in the same manner, and whether they be large or small, startling or insignificant, these changes are all of the nature of mutations, sharp and definite steps from one condition to another. Such actual changes in the nature of the germ-plasm are, however, the exception and not the rule. This conception of variation is quite different from that of Darwin, who believed that inborn variation was the rule, that the characters of a species were in a constant state of flux, so that new variations in one direction or another could be produced merely by selecting in this or that direction. He emphasized the slowness of the process of change in normal life by the aphorism "Natura non fecit saltum." It is true indeed that change in the mass is slow, but to-day we believe that nature does leap, in fact that it is only by leaps that change can occur,