The Case of Neuter Insects.—A very instructive example of the improbability that great skill, highly specialized structures, or certain instincts are first developed in the parental body as the result of use and then passed on to the offspring, is seen in the case of neuter insects. In bees, for example, there are three classes of individuals: the drones or males; the queens or functional females; and the workers, which are neuter, that is, take no part in reproduction. The latter are really sexually undeveloped females. The queen can lay either fertilized or unfertilized eggs. The latter always give rise to males. The workers gather the food, attend the queen, wait on the young, construct the comb, and in short perform all the ordinary functions of the colony except the reproductive. They have many highly specialized structures on various parts of their bodies for carrying on their many activities, as well as the very highly specialized instincts necessary to the maintenance of the colony. But now, complex and highly developed as these workers are, since they do not give rise to offspring, no matter how much experience or structural modifications they may acquire during their lifetime, it can not be handed on to another generation. Nor can they have come to their present highly organized state through such a form of transmission since they are not the descendants of workers but of a queen. Any new modifications that appear in the workers of a colony must therefore have their origin in changes which have taken place in the germ-cells of the queen, and not in the soma of some other worker. It has been argued that the worker has not always been infertile; that at a more primitive stage of the evolution of the bee colony every female was both worker and mother, and that individual somatic acquirements might therefore have been transmitted, but this argument can not hold for many of the instincts or features of the modern bee because these have to do only with the conditions of life which exist in the colony in its present form. It is obviously absurd to maintain, for instance, that all the highly specialized instincts incident to queen production, queen attendance and the like were functionally produced through usage before there was any queen to produce or attend, while on the other hand, the very necessity of queen production and maintenance is the outcome of the infertility of the workers. Some workers have been known to lay eggs, but as these are few in number and are never fertilized, which means if they develop they can only produce males, they can play no considerable part in inheritance.
ORIGIN OF NEW CHARACTERS
Origin of New Characters in Germinal Variation.—This brings us to our last query as to whether the appearance of new characters can be explained on any other or any more inclusive ground than that which infers that changes undergone by the parent-body are in some way registered in the germ-cells so as to be repeated in a certain measure in the body of the offspring. The answer to the question of how inheritable variations do come to appear in offspring if not through changes produced in the body of the parent, is uncertain; nevertheless most biologists believe that they do not have such a somatic origin but arise directly as germinal variations. Some would attribute them to the fluctuating nature of living substance in general. The instability of protoplasm is one of its striking characteristics. It is constantly being broken down and built up, or, in other words, undergoing waste and repair. Like all other protoplasm, that of the germ-cells must also undergo these metabolic changes and it is possible though not proved that in this give and take of substances small changes occur in their constitution which find expression in the offspring as variations. As already seen, substances in the blood other than food may also affect the constitution of the germ-cells.
Sexual Reproduction in Relation to New Characters.—Some biologists attribute great importance to sexual reproduction as a basis of variation and the origin of new characters. They argue that the mingling of determiners from two different lines must produce many new combinations and expressions of germinal potentialities. Plausible as the argument seems at first sight no one has succeeded as yet in securing proof that absolutely new characters can be originated in this way. What seems to occur under such circumstances is merely a reshuffling or sorting of old unit-characters. Although innumerable permutations and combinations of these may be made which find new expression outwardly, this is obviously not creating determiners of new unit-characters in the germ-plasm. While many biologists would not deny the possibility or even the probability that the determiners of unit-characters may sometimes combine or influence one another so as to form actual permanent new characters, the proof of such performance is wholly lacking. On the other hand, there are not a few biologists who argue that sexual reproduction accomplishes just the reverse of increasing the extent of variation or creating new characters; according to them it tends to annul exceptional peculiarities of either parent by throwing the offspring back to the average racial type. It is thus looked on by these advocates as a stabilizer which reduces the amplitude of variations instead of increasing them. As a matter of fact the two ideas are not mutually exclusive; sexual reproduction may accomplish both of these ends. A limited number of observations and experiments have been made to test out the correlation between sexual reproduction and variation, but they have so far been too few or too inconclusive to enable us to come to a satisfactory conclusion.
While we are uncertain about the method of origin of new characters the fact remains that they do arise in abundance as abrupt mutations or otherwise and become a part of the permanent heritage of a stock. It is clear that sexual reproduction may be one important means by which a given new character which has arisen in one or a few individuals may become incorporated in the species at large. Through Mendelian combinations and segregations it would by cross-breeding be spread and gradually introduced into more and more strains of the general population.
Why So Many Features of an Organism Are Characterized by Utility.—Germinal variations are seemingly at first more or less hit or miss affairs as far as utility to the organism is concerned. Useless variations, so long as they are not actually harmful, may persist and apparently be indefinitely inherited. However, a special premium is put on variations which happen to be useful for they help the organism to succeed in its struggle for life and since success in the world of life means not only mere individual survival but also the production of progeny, through this very means insured transmission to subsequent generations. It is probable that the very many useful features of any organism, that is, its adaptations, have thus been established. It is possible also that many variations which at their inception are indifferent may wax in strength in successive generations until they reach a point where they must become either useful or harmful. In the former case they would mean increased insurance of survival for their possessors, in the latter, elimination. With such an automatic process as this operative in nature it is not astonishing that the main features of any organism are characterized by their utility to it.
Germinal Variation a Simpler and More Inclusive Explanation.—The gist of the whole matter regarding the source of new characters in offspring seems to be that the explanation based on the idea of germinal variation is in last analysis the simpler and more inclusive, and there is no alleged case of inheritance of parental modification, which can not be equally well explained as the result of a germinal variation. There are numerous cases which can not be explained as transmissions of somatic acquirements even if this transmission could be established in certain cases. So, many biologists argue, why have two explanations when one is sufficient, especially when the other has never been conclusively established as true in any case and is obviously untrue in certain test cases? The attitude of most investigators is that of the open mind. While feeling that the weight of probability is very decidedly against the theory of the inheritance of somatic modifications, they still stand ready and willing to accept any evidence in its favor which when weighed in the balance is not found wanting.
ANALYSIS OF CASES
While space will not permit extended discussion, in order further to fix the nature of the problem in mind as well as to exemplify the conditions that must be satisfied to form convincing evidence of inherited somatic acquirements, it will be well perhaps to analyze a few typical cases as they are frequently cited.