Page:Popular Science Monthly Volume 63.djvu/230

226 which a few years ago was properly characterized as more Darwinian than Darwin might now be described as more Mendelian than Mendel, and expects to find in 'Mendel's laws' an explanation of heredity, to say nothing of other things. Crosses between some twenty-six close-bred varieties of plants and animals have been found to 'Mendelize,' as the new expression is, and it may be expected that others will do the same wherever the conditions of the experiment can be met, though no amount of similar facts would justify the general conclusions which some recent writers have so promptly drawn. 'Mendel's laws' have already had many different statements, but the most that can be said with certainty is that after close-bred varieties of a plant or animal have sufficiently separated, their divergent characters do not again blend or reduce to an average, but draw apart into definite proportions of each succeeding generation of offspring. Obviously, this is not a method or law of inheritance, but of non-inheritance or fractional inheritance. The sterile and aberrant hybrids are evidence that too wide crossing is not advantageous and makes no contribution to evolutionary progress. Mendel's experiments afford further evidence of the same fact, in that the organisms themselves are found to have means of dissolving such alliances and thus of holding to the paths on which their varietal divergencies have gone forward. The theory that hybridization assists evolution by encouraging variability is shown to have a distinct limit, since little evolutionary progress would come from mere combination of the stable or divergent characters which are a prerequisite of the Mendelian experiments.

Synthetic or Blended Hybrids.—If the normal flexibility of the organism has not been diminished by narrow segregation or inbreeding, the Mendelian repugnance of divergent characters does not appear; Mendel's law of reciprocal characters gives place to Spillman's law of blended or graded characters. Thus there is no record of a normal straight-haired white child as the offspring of two mulattoes. Inbreeding to an extent far beyond anything usual in nature is the rule among domesticated plants and animals, but if the varieties are not too divergent they cross freely and with obvious advantage, as shown by increase in vigor, though such 'new characters' soon disappear under renewed inbreeding. Characters which would become dominant in the Mendelian hybrids are in the less divergent stages termed prepotent, that is, they are impressed with increased intensity upon increasing numbers of each successive generation. On the other