Researches on the Chemical and Mineralogical Composition of Meteorites.

tected, as it were, from another crossing over. This relation is illustrated in the diagram (fig. 7). If crossing over occurs at some point between two pairs of factors a and b indicated on the diagram, the next crossing over point of the chromosome would, if it occurred at all, lie at some distance away, rather than near by. This possibility can be tested in a case where several known points are present in a pair of chromosomes. If in such a case we determine how often crossing over occurs between A and a in general and then determine how often it occurs between A and a in those cases where it is known to have taken place between a and b, we find an enormous decrease in the number of times it occurs between A and a when at the same time it has occurred between a and b. In general one may say that crossing over at any level interferes with crossing over in the region of each side of that level. Conclusions. The chromosomes not only furnish a mechanistic explanation of Mendelian heredity, but in the case of Non-disjunction and in the case of the point by point correspondence between the Linkage Groups and the chromosomes, furnish a verifiable explanation of the results. In the case of Crossing-over and of Interference the chromosomes give us the only objective explanation of the results that has been as yet offered.