Genetics has adduced cogent evidence that, despite the strong influence of the environment in modifying the body as a whole, and even the protoplasm of its cells, the genes within the germ cells of that body retain their original structure without specific alterations caused by the modification of the body, so that when the modified-individual reproduces it transmits to its offspring genes unaffected by its own "acquired characters." The offspring, then, will not tend to repeat the parental modifications, unless the same peculiar environment is itself repeated. Our sins and our successes are not reborn in our children, though the latter ntay inherit our original tendency and capacity for these. Modifica tions, therefore, unlike mutations, cannot be a cause of biological evolution or degeneration. Now, if modifications acquired before the conception of children are not visited upon the latter, it cer tainly holds true that any modifications acquired by the mother after the embryos are partly developed, but prior to their birth, would not be transmitted to them. Mothers may rest assured that there is no more ground either a priori or in observation, for sup posing that a fright received during pregnancy (or before) will give their child a fearsome disposition than that it will give him spots.
Variation Due to Segregation and Recombination of Genes.—Whenever an individual breeds, having received a dif ferent hereditary contribution from its two parents, its offspring will tend to vary from one another. Thus, if we had received a gene for brown eyes from our mother, and one for blue eyes from our father, then, although we ourselves would have brown ish eyes (brown being said to be "dominant" and blue "reces sive"), nevertheless we would transmit our gene for brown only to half of our reproductive cells, and the gene for blue, unalloyed by brown, to the remainder. This process of separation is called segregation (see HEREDITY. If our parents had given us dif ferent genes for hair-form also, we would produce reproductive cells having all the possible assortments of the two pairs of genes, namely, some having "brown curly," others "blue straight," still others "brown straight" and the rest "blue curly." This phe nomenon is recombination. Since such variation can be produced in our children only in respect to genes which already differed from one another in our parents, it is evident that the primary cause of this variation lies in the events whereby these genes came to differ in the first place, that is, in the gene-mutations such as originally changed a gene for brown eyes into the gene for blue. These mutations, that make possible the recombinations of to-day, may have happened in the remote past.
The explanation of segregation and recombination is to be found in the behaviour of the chromosomes, those tiny separate filamentous bodies, visible under the microscope, which contain the invisible genes "linked" together within them, like beads in so many separate chains. Several or many chromosomes, each
probably containing hundreds of different genes of distinctive natures and effects, are contained in each of the two uniting repro ductive cells derived from the two parents. While the majority of the genes from one parent are always like those from the other, since the parents are in general similar organisms, yet in the case of most matings there are some gene-differences, and these give an opportunity for the operation of the segregation and recombination processes above mentioned, after the individual formed by the union of the two cells in question develops and reproduces reproductive cells for a still later generation.
It will be seen that the more gene-differences exist the more different combinations can then be formed_ The numerical rela tions resulting from recombination are calculable according to definite laws, provided the effects of the genes concerned, and their positions in the chromosomes, have been determined by prior experimentation. The character-effects of untried combinations cannot, however, be predicted with certainty, as unexpected de velopmental results, including even new traits, are occasionally produced by recombination. As some new combinations may be more advantageous for the race than the original combinations, the function of recombination, of segregation, and in fact of sex ual reproduction itself, becomes explained in terms of their value in evolution.
The development of most characters depends on the combined action of numerous genes. A difference in any one of these genes may affect either the quality, or the degree of development of the character in question. Now, when a number of mutations, in dif ferent genes all concerned with the same character (e.g., stature), have some time previously occurred in a population, random crossing and recombination will result in the transmission, to dif ferent offspring, of many different assortments of the mutated and non-mutated genes. Thus the population may tend to ex hibit many different grades of expression of the character. These grades will often differ only by small steps, which become blurred into an apparently continuous gradient through the effect of en vironic modifications. Such quantitative variation, as well as all other variation of recombinational origin, is of course especially evident when individuals are bred which resulted from the crossing of widely different stocks, in which many differing gene mutations had become established since these stocks diverged from their common ancestor.