In the formation of the germ cells in sexual reproduction, the idioplasms of both parents must come into contact with each other, whereupon there results either a material union and formation of a mixed idioplasm or perhaps rather a dynamic action; and through these agencies there is produced a remodeled form which is, however, exactly equivalent to the combined idioplasms entering into it. Fertilization by diosmose of the spermatic substance is impossible.[D]
In the idioplasm of a germ cell arising from the crossing of unlike individuals the micellar rows of the individual determinants have sometimes an intermediate constitution and produce characteristics in the organism which are intermediate between the characteristics of the parents. Sometimes the micellar rows derived from the father and mother respectively lie side by side unchanged in the idioplasm of the offspring in distinct groupings and may reproduce in the organism their respective characteristics side by side, or only one of them may develop, while the other remains latent.
On account of the union of both idioplasms as the result of fecundation, two sexually mature organisms are the more able to form with each other a viable germ cell, the nearer they are genetically related—that is, the more nearly the male and female idioplasms correspond in their configuration and chemical nature, because in this case the micellar arrangements are best suited to each other, and the idioplasm of the new fertile germ cell receives its most suitable nourishment from the mother. If, however, self-fecundation or the closest in-and-in breeding often yields products of less virility and is avoided by nature, this is the result of injurious influences which make themselves felt later on. This is because incompatibilities may be present in too closely related idioplasms and these are sources of weakness in unrestricted development. The more complicated is the idioplasm, the oftener this occurs, whereas absolute lack of crossing is not detrimental to the simplest (asexual) organisms.
11. ACTION OF EXTERNAL INFLUENCES.[E]
The environment provides the organism above all with force and matter for its life processes. It causes no permanent variation and has only an ontogenetic significance, if the limits of the idioplasmic elasticity are not exceeded; it maintains the growth and metabolic assimilation of the individual, and conditions individual (not hereditary) differences, which constitute "nutrition varieties." (See page 30.) These appear as the direct results of operating causes.
When the stress of environment exceeds the limits of idioplasmic elasticity, its influence brings about permanent variations, which are imperceptibly small, it is true, in the single individual, but which, when the stimulus is active for a long period of time in the same manner, increase to perceptible magnitude. These variations are inheritable in the phylogenetic sense and contribute to the formation of varieties and species; they always appear as the results of more or less secondary reactions which make their appearance with stimuli exerted by external causes.
External stimuli exerted on the organism are reproduced in the idioplasm. Since the stimulus is discontinued with each change of the ontogeny and only the idioplasm persists, permanent variations are produced only in the idioplasm by those conditions that produce visible transformations in the mature organism.
The phylogenetic action of external stimuli gives the definite character of adaptation to the idioplasm as it becomes more complex from inner causes and probably these external stimuli have the power to alter this impress only as new idioplasm is automatically formed.
If an external cause acts continuously upon a phylogenetic line, the corresponding variation of the idioplasm reaches, after a time, a maximum, and thus comes to an end, either because the nature of the substance permits no new rearrangement or because the stimulus is no longer active. The cessation of the stimulus results from a micellar rearrangement which indicates the character of the adaptation. If the action of the stimulus lasts for only a short time, the incipient rearrangement of the idioplasm stops, or proceeds independently on account of the impulse received, and the determinant becomes capable of development, even after the impulse has long ceased to act.
Since various intervening transpositions follow upon a stimulus in the organism, the final result which appears as a reaction may turn out variously. The same external causes may, according to the nature of the organism and other circumstances, have very unlike variations as a result. But the internal rearrangement produces in a definite case very definite variations.