A Critique of the Theory of Evolution

by Thomas Hunt Morgan

Furthermore, from Castle's point of view, these latest results would seem to increase the difficulty of interpretation of his first F₂ extracted cross, and it is now the first result that calls for explanation if one accepts his later conclusion.

These and other objections that might be taken up show, I think, that Castle's

experiment with hooded rats fails entirely to establish his contention of change in potency of the germ or of contamination of factors, while on the contrary they are in entire accord with the view that he is dealing with a case of modifying factors.

Fig. 89. Races of Paramecium. (After Jennings.)

Equally important are the results that Jennings has obtained with certain protozoa. Paramecium multiplies by dividing across in the

middle, each half replacing its lacking part. Both the small nucleus (micronucleus) and the large nucleus (macronucleus) divide at each division of the body. Jennings found that while individuals descended from a single paramecium vary in size (fig. 89), yet the population from a large individual is the same as the population derived from a small individual. In other words, selection produces no result and the probable explanation is, of course, that the different sizes of individuals are due to the environment, while the constancy of the type is genetic. Jennings found a number of races of paramecium of different sizes living under natural conditions. The largest individual of a small race might overlap the smallest individual of other larger races (fig. 89); nevertheless each kind reproduced its particular race. The results are like those of Johannsen in a general way, but differ in that reproduction takes place in paramecium by direct division instead of through self-fertilization as in beans, and also in that the paramecia were probably not homozygous. Since, however, so far as known no "reduction" takes place in

paramecium at each division, the genetic composition of parent and offspring should be the same. Whether pseudo-parthenogenesis that Woodruff and Erdmann have found occurring in paramecium at intervals involves a redistribution of the hereditary factors is not clear. Jennings's evidence seems incompatible with such a view.

Fig. 90. Stylonychia showing division into two. (After Stein.)